CHAPTER 15 • Placenta and Umbilical Cord. First Trimester Ultr

 CHAPTER 15 • Placenta and Umbilical Cord

INTRODUCTION

The placenta is a highly specialized organ that supports growth and development of the

fetus and serves as the interface between the maternal and fetal circulations. The

placenta functions as the pregnancy organ that delivers nutrients, exchanges respiratory

gas, and eliminates toxic waste. The placenta is also an important endocrine organ

producing hormones to support and sustain pregnancy and plays a critical role in

prevention of pregnancy rejection. Impairment in placental development and/or function

has a profound impact on pregnancy outcome. Accumulating data suggest that the

placenta plays a critical role in the future health of the fetus such as the risk for adultonset cardiovascular disease among other diseases.1–4 This chapter presents the

embryologic development and normal sonographic appearance of the placenta and

umbilical cord and discusses common placental and cord abnormalities that can be

detected in the first trimester of pregnancy.

EMBRYOLOGY

The blastocyst reaches the endometrial cavity at 4 to 5 days postfertilization. The outer

surface of the blastocyst differentiates into trophoblastic cells and produces an

overlying syncytial layer that adheres to the endometrium. Implantation of the blastocyst

then commences as the syncytiotrophoblast cells penetrate the decidualized

endometrium. Endometrial gland secretions provide nourishment to the embryo at this

early stage. Spaces are then developed within the syncytiotrophoblast and form

anastomosis with maternal vascular sinusoids, thus establishing the first (lacunar)

uteroplacental circulation (Fig. 15.1). The placental circulation then develops with

finger-like projections into the maternal blood spaces. These projections extend from

the chorion and form the primary villi with an inner layer of cytotrophoblast and an

outer layer of syncytiotrophoblast. The primary villi become secondary villi with the

invasion of the extraembryonic mesoderm and finally become tertiary villi as embryonic

blood vessels develop within them. In the early stages of placental development,cytotrophoblasts invade the endothelium and smooth muscle of endometrial spiral

arteries, releasing them from maternal influences. The fully formed human placenta is

termed hemochorial because the maternal blood is separated from the fetal blood only

by elements of the chorion. Growth in size and thickness of the placenta continues

rapidly in the first trimester and into the second trimester of pregnancy. The term

placenta has a fetal portion, the chorion frondosum, and a maternal portion, the decidua

basalis, and covers 15% to 30% of the decidua of the endometrial cavity.5 The placenta

at term is about 20 cm in diameter, has a volume of 400 to 600 mL, and weighs

approximately one-sixth as much as the fetus.5,6

Figure 15.1: Schematic drawing of early stages of establishment of the first

(lacunar) uteroplacental circulation, shortly after implantation of the blastocyst.

Note the presence of spaces developed within the syncytiotrophoblast and

forming anastomosis with maternal vascular sinusoids. See text for details.Figure 15.2: Schematic drawing of the embryogenesis of the umbilical cord.

The umbilical cord is formed by fusion of the connecting stalk (allantois and

umbilical vessels) and the vitelline duct and vessels with cephalocaudal flexion

of the embryo. See text for details.

In early embryogenesis, two stalks are seen: the yolk sac, ventrally located and

containing the vitelline duct and vessels, and the connecting stalk, caudally located and

containing the allantois and the umbilical vessels (Fig. 15.2). With cephalocaudal

flexion of the embryo, the connecting stalk fuses with the yolk sac stalk to form the

umbilical cord. The umbilical cord in its early development is inserted in the lower

ventral portions of the embryo, is short and thick, and contains the allantois, the vitelline

duct and vessels, and the umbilical vessels. The amnion covers the umbilical cord and

becomes continuous with the outer epithelial layer of the embryo. The umbilical cord

elongates and thins out with the development of the anterior abdominal wall. The

umbilical cord initially attaches centrally to the developing placenta. As the placenta

grows, it tends to expand preferentially in regions with sufficient myometrial perfusion

and atrophy in areas with suboptimal blood supply. As a result, the cord insertion may

become somewhat eccentric. This process is known as trophotropism. The umbilical

cord consists of two umbilical arteries and one vein, which are surrounded by mucoid

connective tissue—Wharton jelly. The cord at term is usually 1 to 2 cm in diameter and

30 to 90 cm in length.Figure 15.3: Two-dimensional ultrasound in two pregnancies (A and B) at 9

weeks of gestation demonstrating the appearance of the placenta. Note that

the placenta is slightly more echogenic than the surrounding endometrium. The

decidua (endometrium behind the placenta) is hypoechogenic in appearance.Figure 15.4: Two-dimensional ultrasound in two pregnancies at 10 (A) and 12

(B) weeks of gestation, respectively, demonstrating the appearance of the

placenta. Note that the placenta in A and B has a uniform homogeneous

echotexture and is slightly more echogenic than the surrounding endometrium

and uterine wall. Placental cord insertion is shown in B.

NORMAL SONOGRAPHIC ANATOMY

The placenta is first recognized sonographically as a thickened echogenic region on the

endometrium by about 9 to 10 weeks of gestation (Figs. 15.3 and 15.4A). By 12 to 13

weeks of gestation, the placenta is easily seen on ultrasound and appears slightly

echogenic with uniformed homogeneous echotexture (Figs. 15.4B and 15.5).

Figure 15.5: Two-dimensional ultrasound in two pregnancies (A and B) at 13

weeks of gestation, demonstrating the appearance of the placenta. A: Image

obtained by the transabdominal approach. B: Image obtained by the

transvaginal approach. Note that the transvaginal approach clearly outlines

placental borders because of increased resolution of the transducer. Placental

cord insertion is shown in B.Figure 15.6: Placental location in the first trimester is best described after

localizing the cervix (arrows) and anterior/posterior uterine walls.

Transabdominal ultrasound in two pregnancies (A and B), where the placentas

appear to be on the anterior uterine walls. In A, the placenta is anterior;

however, in B, because of the presence of uterine anteflexion, the posterior

uterine wall is seen as closest to the transducer, and thus the placenta is

posterior. Inf, Inferior; Sup, superior.

During the first trimester ultrasound examination (see Chapter 5), the location of the

placenta in the uterus should be reported. Identifying the location of the placenta on the

first trimester ultrasound is not as easy as in the second trimester given the presence of

uterine flexion and extension. Indeed, inaccuracies can be introduced especially when

the placenta appears to be in the lower uterine segment. In order to improve accuracy of

placental localization on the first trimester ultrasound, we recommend identifying the

cervix and the anterior and posterior uterine walls before describing the placental

location (Figs. 15.6 and 15.7).

The placental size, thickness, location within the endometrial cavity, and

echogenicity can also be evaluated by ultrasound in the late first trimester of

pregnancy.7 The normal thickness of the placenta is correlated to gestational age of the

embryo/fetus and is approximately 1 mm per week of gestation.8 With advanced threedimensional (3D) ultrasound techniques, placental volume can now be measured in the

first trimester (Fig. 15.8), and along with placental biometry has been shown to

correlate with pregnancy complications.9,10 Indeed, we believe that first trimester

measurement of placental length, width, and volume is more accurate than later on in

pregnancy given that the whole placenta is seen in one ultrasound image in earlygestation. The assessment of biometric dimensions of the placenta is infrequently

performed on prenatal sonography today, unless in rare pathologic conditions or for

research purposes. Abnormal placental findings on first trimester ultrasound, such as

masses, multiple cystic spaces, or large subchorionic fluid collection, should be noted

and followed up.

Figure 15.7: Placental location in the first trimester is best described after

localizing the cervix (arrows) and anterior/posterior uterine walls.

Transabdominal ultrasound in two pregnancies (A and B), where the placentas

appear to be on the posterior uterine walls. In A, the placenta is anterior, and

the presence of uterine anteflexion gives an erroneous impression of a

posterior location of the placenta. In B, the placenta is clearly posterior in

location. Inf, Inferior; Sup, superior.Figure 15.8: Three-dimensional (3D) ultrasound volume of a placenta at 13

weeks of gestation shown in multiplayer display with placental volume obtained

from the analysis of the 3D volume set. The placental volume is 88.58 cm3.

Maternal blood flow is established within the placenta by 12 weeks of gestation.11

Blood flow in the maternal and fetal placental vasculature can be demonstrated in the

first trimester on color and pulsed Doppler ultrasound (Fig. 15.9). Quantitative

assessment of placental vascularization may be useful for predicting pregnancy

complications and adverse events.12–14Figure 15.9: A: Two-dimensional ultrasound in color Doppler of a placenta in a

pregnancy at 12 weeks of gestation demonstrating the maternal and fetal

circulation at the level of the cord insertion. B and C: The corresponding pulsed

Doppler of the maternal and fetal vasculature. Note the differences in blood

flow velocities between the maternal and fetal circulation, with the maternal

circulation showing a low impedance pattern. Also note the difference in heart

rates between the two circulations.Figure 15.10: Three-dimensional ultrasound in surface mode (A, C–E) and

two-dimensional ultrasound (B) of the embryo and umbilical cord at 7 to 10

weeks of gestational age. Note that the umbilical cord in this gestational age

window is short and thick and connects the embryo to the placenta. Note in E,

at 10 weeks of gestation, thickening of the umbilical cord at the abdominal cord

insertion (asterisk), corresponding to the physiologic hernia.

Figure 15.11: Two-dimensional ultrasound in gray scale (A) and color Doppler

(B) of the umbilical cord in a pregnancy at 13 weeks of gestation. Note in A and

B that the umbilical cord is elongated and thinned from its appearance between

7 and 10 weeks of gestation (Fig. 15.10). The umbilical cord at 13 weeks of

gestation has the same appearance as that in the second trimester of

pregnancy.Figure 15.12: A: An axial plane (cross-section) of the pelvis in color Doppler in

a fetus at 13 weeks of gestation demonstrating the two umbilical arteries (UA)

surrounding the fetal bladder. B: A midline sagittal plane in color Doppler in a

fetus at 12 weeks of gestation demonstrating the umbilical cord insertion into

the fetal abdomen.

The umbilical cord can be recognized by ultrasound as early as the seventh week of

gestation and appears as a straight thick structure connecting the embryo to the

developing placenta (Fig. 15.10). In the first trimester, the length of the umbilical cord

is approximately the same as the crown-rump length.15 The umbilical cord elongates and

thins with advancing gestation, and by the 13th week, the cord has the same sonographic

appearance as in the second trimester of pregnancy (Fig. 15.11). Umbilical arteries can

be seen in the first trimester as branches of the internal iliac arteries, running alongside

the fetal bladder in a cross-section view of the fetal pelvis using color or power

Doppler (Fig. 15.12A), and the number of umbilical arteries can thus be reliably

determined.16 Midline sagittal plane of the fetus demonstrates the insertion of the

umbilical cord into the abdomen, and this plane is also important in the evaluation of the

integrity of the abdominal wall in the first trimester (Fig. 15.12B) (see Chapters 5 and

12) . 3D ultrasound in surface mode can also clearly demonstrate the umbilical cord

(Fig. 15.13).Figure 15.13: Three-dimensional ultrasound in surface mode in two fetuses at

12 (A) and 13 (B) weeks of gestation demonstrating external fetal anatomy

along with the umbilical cord (arrow).

Figure 15.14: Two-dimensional ultrasound in a pregnancy at 9 weeks of

gestation demonstrating a subchorionic hematoma (asterisk and arrows)

located between the chorion and the uterine wall.

PLACENTAL ABNORMALITIES

Intrauterine Hematoma

Intrauterine hematoma is a common finding on routine ultrasound in the first trimester,especially among pregnant women presenting with vaginal bleeding. Intrauterine

hematoma usually appears as a crescent-shaped, sonolucent fluid collection behind the

fetal membranes or the placenta, but may vary significantly in shape and size. The

position of the hematoma relative to the placental site can be described as subchorionic

or retroplacental. The subchorionic hematoma is located between the chorion and the

uterine wall (Fig. 15.14), whereas the retroplacental hematoma is located behind the

placenta (Fig. 15.15). The reported incidence of first trimester hematomas diagnosed by

ultrasound varies widely, from as low as 0.5% to as high as 22%, depending on the

patient population studied and the ultrasound evaluation.17,18 In low-risk general

obstetric population, intrauterine hematomas occur in 3.1% of cases in the first

trimester.19 Although approximately 70% of subchorionic hematomas resolve

spontaneously by the end of the second trimester without clinical sequelae, some may

persist until the end of pregnancy and be associated with increased risk of pregnancy

complications.17

The clinical significance of an intrauterine hematoma noted on the first trimester

ultrasound is currently controversial.20–24 Systematic review and meta-analysis

performed by Tuuli et al.25 demonstrated that presence of a first trimester intrauterine

hematoma is associated with adverse pregnancy outcome, including an increased risk

for spontaneous abortion, stillbirth, placental abruption, preterm premature rupture of

membranes, and preterm delivery. There is no consistency in study results, however,

and the association of an intrauterine hematoma with pregnancy complications such as

preeclampsia and fetal growth restriction has not been confirmed.25 Retroplacental

position of the hematoma (Fig. 15.15) appears to carry a higher risk for poor pregnancy

outcome.19 An intrauterine hematoma can be classified based upon its relative size. In a

study on this subject, the size of the hematoma was graded according to the percentage

of chorionic sac circumference elevated by the hematoma, with small indicating less

than one-third of the chorionic sac circumference, moderate indicating one-third to onehalf of the chorionic sac circumference, and large indicating two-thirds or greater of

chorionic sac circumference.22 Large intrauterine hematomas (Fig. 15.16) were found to

be associated with an almost threefold increase in risk of spontaneous abortion.22 There

is currently sufficient evidence in the literature to suggest that the presence of a first

trimester large intrauterine hematoma may increase the pregnancy risk, and follow-up

ultrasound examinations in the second and possibly third trimester of pregnancy is thus

warranted.

Although a subchorionic hematoma is relatively easy to identify in the first trimester,

the diagnosis of a subplacental hematoma is challenging especially in the absence of

clinical symptoms. The presence of a transient uterine contraction (see Fig. 5.1) or

thickened uterine wall can mimic the diagnosis of subplacental bleed. The application

of color Doppler can help differentiate a subplacental bleed from a uterine contractionor thickening.

Figure 15.15: Two-dimensional ultrasound with color Doppler in a pregnancy at

10 weeks of gestation demonstrating a retroplacental hematoma (asterisk and

arrows) located behind the placenta.

Figure 15.16: Two-dimensional ultrasound in a pregnancy at 10 weeks of

gestation demonstrating a large intrauterine hematoma (asterisk and arrows).

Note that the size of this hematoma (color overlay) is almost larger than the

circumference of the gestational sac.Figure 15.17: Two-dimensional transvaginal ultrasound in a pregnancy at 12

weeks of gestation demonstrating a placenta previa. Note that the placenta is

covering the internal cervical os.

Placenta Previa

The term placenta previa describes a placenta that covers the internal cervical os. In

normal pregnancy, the placenta implants in the upper uterine segment. In the case of

placenta previa, the placenta is partially or totally implanted in the lower uterine

segment and placental tissue covers the internal cervical os (Figs. 15.17 and 15.18). In

the second trimester of pregnancy, if the placenta is attached in the lower uterine

segment and placental tissue does not cover the internal os, but is within 2 cm from the

internal os, the placenta is called low lying.Figure 15.18: Two-dimensional transvaginal ultrasound in a pregnancy at 13

weeks of gestation demonstrating a placenta previa. Note that the placenta is

covering the internal cervical os.

The incidence of placenta previa varies greatly with gestational age. Placenta previa

is more commonly seen in early gestation and presents in approximately 4.5% to 6.2%

of pregnancies between 12 and 16 weeks of gestation.26,27 The proportion of patients

with the placenta extending to or covering the internal cervical os significantly

decreases with advancing gestation from 5.5% at 12+0 to 12+6 weeks, to 2.4% at 15+0

to 15+6 weeks of gestation and to 0.16% at term.26 Several studies have shown that if

the placenta extends at least 15 mm over the internal cervical os at 12 to 16 weeks of

gestation, a placenta previa is present at term with a sensitivity of 80% and a positive

predictive value of 5.1%.26–28 The mechanism resulting in the resolution of a first

trimester placenta previa with advancing gestation is poorly understood, but may be

related to a preferential growth of the placenta toward a better vascularized upper

endometrium (trophotropism). According to current guidelines for performance of first

trimester fetal ultrasound, it is not recommended to report the presence of placenta

previa or low-lying placenta between 11+0 and 13+6 weeks of gestation because the

position of the placenta in relation to the cervix at this stage of pregnancy is of less

clinical importance as a result of the “migration” phenomenon.7

Morbidly Adherent PlacentaThe term “morbidly adherent placenta” implies abnormal implantation of the placenta

into the uterine wall, and this term has been used to describe placenta accreta, increta,

and percreta. Placenta accreta occurs when the placental villi adhere directly to the

myometrium, a placenta increta involves placental villi invading into the myometrium,

and a placenta percreta is defined as placental villi invading through myometrium and

into serosa and, sometimes, adjacent organs. About 75% of morbidly adherent placentas

are placenta accretas, 18% are placenta incretas, and 7% are placenta percretas,29 but

this differentiation is not always possible on prenatal ultrasound. We will use the term

placenta accreta to describe morbidly adherent placenta.

The sonographic markers of placenta accreta in the first trimester primarily include a

gestational sac that is implanted in the lower uterine segment (Fig. 15.19), a gestational

sac that is embedded in a cesarean section scar (Figs. 15.20 and 15.21) (cesarean scar

pregnancy), and the presence of multiple vascular spaces (lacunae) within the placental

bed, primarily in the setting of a placenta previa (Figs. 15.22 and 15.23).

Ballas et al.30 defined lower uterine segment implantation as a gestational sac that is

implanted in the lower third of the uterus between 8 and 10 weeks or primarily

occupying the lower uterine segment from 10 weeks (Fig. 15.19). In the authors’

experience, identifying that a gestational sac is in the lower uterine segment is more

difficult at 10 weeks of gestation and beyond as the gestational sac typically expands

into the upper uterine segment. One must also differentiate lower uterine segment

implantation from an ongoing pregnancy loss (miscarriage). With the application of

color Doppler, a failing pregnancy can be clearly distinguished as a sac that lacks

circumferential blood flow, in addition to a sac that moves when pressure is applied to

the anterior surface of the uterus.31 Not all gestational sacs that implant in the lower

uterine segment lead to placenta accretas, because subsequent normal pregnancies in

this setting have been reported.32 In these instances, a normal thick anterior myometrium

superior to the gestational sac and a continuous white line representing the bladder–

uterine wall interface is seen on ultrasound. The gestational sac should be contiguous

with the endometrial cavity.31Figure 15.19: Two-dimensional transvaginal ultrasound in a pregnancy at 7

weeks of gestation demonstrating a low implantation of the gestational sac.

Note that the gestational sac is in the lower uterine segment, posterior to the

bladder and next to the cervix. This patient had three prior cesarean deliveries

and the placenta was diagnosed as placenta previa and accreta in the second

and third trimester of pregnancy.

In patients with a prior cesarean section, pregnancies implanted in or near the

cesarean section scar (Figs. 15.20 and 15.21) carry significant risk for placenta accreta

and pregnancy complications. In these cases, the gestational sac appears embedded into

the cesarean section scar, the anterior myometrium appears thin, and the placental–

myometrial and bladder–uterine wall interfaces often appear irregular.31 In the authors’

experience, the gestational sac of a cesarean scar implantation is typically fusiform in

shape at 6 to 8 weeks of gestation (Fig. 15.21). Color Doppler reveals increased

vascularity surrounding the gestational sac (Fig. 15.21B).Figure 15.20: Transvaginal ultrasound of the midline sagittal plane of the uterus

in a pregnancy at 7 weeks of gestation demonstrating a cesarean section scar

implantation. Note that the gestational sac is imbedded into the cesarean

section scar. Also note the proximity of the empty bladder to the gestational

sac.

Many studies combine cesarean scar pregnancies with pregnancies that implant in

the lower uterine segment, near the cesarean section scar.31,33–35 A true cesarean scar

pregnancy is defined by the presence of a gestational sac that is implanted within the

myometrium, surrounded on all sides by myometrium, and be separate from the

endometrium (Figs. 15.20 and 15.21).Figure 15.21: Transvaginal ultrasound of the midline sagittal plane of the uterus

in gray scale (A) and color Doppler (B) in a pregnancy at 7 weeks of gestation

demonstrating a cesarean section scar implantation. Note that the gestational

sac is imbedded into the cesarean section scar. Also note the fusiform shape

of the gestational sac in A and B. Color Doppler shows increased vascularity

surrounding the gestational sac.

Figure 15.22: Transvaginal ultrasound in a pregnancy at 11 weeks of gestation

demonstrating the presence of multiple placental lacunae in a patient with twoprior cesarean sections. The presence of placental lacunae in the first trimester

increases the risk for placenta accreta.

The third marker of placenta accreta in the first trimester is the presence of anechoic

areas within the placenta with or without documented blood flow on color Doppler

(Figs. 15.22 and 15.23). These anechoic areas have been described as vascular spaces,

lacunae, or lakes. Multiple case reports describe the presence of hypoechoic placental

vascular spaces on ultrasound at less than 12 weeks of gestation and have linked their

presence to the early diagnosis of placenta accreta.30,36–39 Three examples of irregularly

shaped placental lacunae diagnosed at 8, 9, and 12 weeks, respectively, were reported

in women presenting with vaginal bleeding and suspicion for abnormal

placentation.36,38,39 Two resulted in hysterectomy secondary to hemorrhage as early as

15 weeks, and placenta accreta was confirmed on pathology. In the third case, the

patient elected termination, and the uterus was preserved. A retrospective study by

Ballas et al.30 further confirmed lacunae as a first trimester marker for placenta accreta.

They reported on 10 cases of placenta accreta with first trimester ultrasound

examinations and noted that anechoic placental areas were present in 8 of 10 (80%).30 If

the pregnancy progresses, these lacunae become more prominent in the second and third

trimester of pregnancy and may demonstrate blood flow on low-velocity color Doppler.

Figure 15.23: Transvaginal ultrasound in a pregnancy at 13 weeks of gestation

demonstrating the presence of multiple placental lacunae and placenta previa in

a patient with one prior cesarean section. The presence of placenta previa with

multiple lacunae in the first trimester increases the risk for placenta accreta.

Amniotic Band Syndrome

It is commonly accepted that amniotic band syndrome (ABS) occurs when the inner

membrane (amnion) ruptures without injury to the outer membrane (chorion), thusexposing the embryo/fetus to fibrous sticky tissue from the ruptured amnion (bands),

which can float in the amniotic fluid. These bands can entangle the fetus, reducing blood

supply and causing a variety of fetal congenital abnormalities. The incidence of ABS is

approximately 1 in 1,200 live births.40 Defects caused by ABS range from minor defects

of the digits to major complex multiorgan anomalies. The most common findings are

constriction rings with lymphedema around the fingers, toes, arms, or legs. ABS should

be suspected in case of limbs amputations and in the presence of unusual asymmetric

craniofacial (Fig. 15.24) or visceral defects. The direct ultrasound visualization of

amniotic bands is challenging and requires high-resolution transducers, preferably by

the transvaginal approach (Fig. 15.24). Use of transvaginal 3D/four-dimensional

imaging can be particularly helpful in the first trimester for the differential diagnosis of

amniotic bands and related fetal abnormalities.41,42 There are several case reports on

the diagnosis of ABS in the first trimester.41–44

CORD ABNORMALITIES

Abnormalities of the umbilical cord are common and may affect the length, size,

number, insertion, and course of the umbilical vessels. The use of color Doppler is very

helpful in the diagnosis of cord insertion and presence of structural abnormalities of the

umbilical cord, between 11 and 14 weeks of gestation.45 Abnormally short umbilical

cord can occur as a result of embryonic infolding failure, which is associated with

limb-body-stalk anomalies (see Chapter 13 for details). Abnormally long cord may

predispose to cord prolapse, nuchal cord, or true cord knots. True cord knots occur in

about 1% of single pregnancies and very rarely can be observed on the first trimester

ultrasound (Fig. 15.25). Color Doppler and 3D ultrasound can help confirm the

presence of a true knot, when suspected, on gray scale ultrasound in the first trimester

(Fig. 15.25).

Cord entanglement is a common complication of the monochorionic–monoamniotic

gestation and can be noted as early as 12 to 13 weeks of gestation. The application of

color and pulsed Doppler can confirm the diagnosis of cord entanglement in the first

trimester of pregnancy (Fig. 15.26A) (see Chapter 7). 3D ultrasound can also confirm

the diagnosis and display the entangled cords (Fig. 15.26B).Figure 15.24: Transvaginal two-dimensional (A) and three-dimensional

ultrasound (B) of a fetus at 13 weeks of gestation with severe brain

malformation (anencephaly—asterisk) resulting from amniotic band syndrome.

Note the presence in A of a reflective membrane within the amniotic cavity

(arrow) that is attached to the fetal head. This reflective membrane represents

an amniotic band.

Excessive or absent coiling of the umbilical cord can be occasionally detected in the

first trimester ultrasound. Abnormal insertion of the umbilical cord may result in

velamentous cord insertion or vasa previa. Abnormally thickened umbilical cord can be

observed in association with fetal hydrops or cord cysts. In the following sections,

common umbilical cord abnormalities are discussed in more detail.

Single Umbilical Artery

The absence of one of the arteries in the umbilical cord is called single umbilical artery

(SUA) or two-vessel umbilical cord. The pathogenesis of SUA is uncertain. Aplasia or

atrophy of the missing vessel has been suggested as an etiology of SUA.46 SUA is one of

the most common sonographic findings during pregnancy and is more commonly seen in

multiple gestations, in the presence of velamentous cord insertion, in advanced maternal

age, in maternal diabetes, in hypertensive and seizure disorders, and in smoking.47,48

The incidence of SUA reported in the first trimester of pregnancy is 1.1% and 3.3% in

single and twin gestations, respectively.45 Detection of SUA in the first trimester

ultrasound is possible, with reported sensitivity between 57.1% and 84.2% andspecificity 98.9% and 99.8%.45,49 This is best performed by obtaining an axial plane of

the fetal pelvis in color Doppler and identifying a SUA next to the bladder (Fig. 15.27),

rather than the two umbilical arteries normally seen (Fig. 15.12A).

Figure 15.25: Two-dimensional (A) and three-dimensional (3D) (B) ultrasound

in color Doppler in a fetus at 12 weeks of gestation with a true cord knot. Note

in A the presence of a thickening of the cord on color Doppler. The presence of

cord thickening (circle) suggests the presence of a true knot. B: A 3D volume

display of the cord in glass body mode demonstrating the cord knot.Figure 15.26: A: Color and pulsed Doppler of cord entanglement in a

monoamniotic twin pregnancy at 13 weeks of gestation. Note the presence of

“a mass of cord” on color Doppler suggesting the diagnosis. Pulsed Doppler

applied to the mass of cord and displayed in lower panel A shows two

interposed fetal umbilical Doppler spectrums (A and B in Doppler spectrum),

confirming cord entanglement. B: A three-dimensional ultrasound in surface

mode of another monoamniotic twin pregnancy at 12 weeks of gestation with

cord entanglement. C: The entangled cords of pregnancy in B after delivery at

33 weeks of gestation.Figure 15.27: Axial plane of the pelvis in color Doppler in two fetuses (A and

B) at 13 weeks of gestation with single umbilical artery. Note in A that the right

umbilical artery (UA) is absent and in B, the left UA is absent. Although earlier

studies suggested a significance to the laterality of single umbilical artery, this

has not been proven in subsequent studies.

Figure 15.28: Gray scale (A) and color Doppler (B) of a pregnancy at 11

weeks of gestation with velamentous insertion of the umbilical cord. Note thatthe umbilical cord inserts in the membranes, rather than directly into the

placenta.

The association of SUA with fetal anomalies, mainly genitourinary and cardiac, as

well as with a wide range of genetic syndromes and chromosomal aberrations has been

reported50–52 and described in various chapters of this book. SUA can also be

associated with intrauterine growth restriction, preterm delivery, and poor pregnancy

outcomes.53 The presence of a SUA as an isolated sonographic finding, however, is

often associated with a normal pregnancy outcome.53–55 A study performed by MartínezPayo et al.45 demonstrated that about 17.6% of SUA cases diagnosed between 11+0 and

13+6 weeks of gestation had concomitant malformations detected at the first trimester

ultrasound, and in an additional 7.7% of SUA cases, anomalies were found in the

second trimester of pregnancy. The authors concluded that the assessment of the number

of umbilical cord vessels during the first trimester ultrasound examination is useful

given the association of SUA with fetal malformations that can be diagnosed in early

gestation.45 We recommend a detailed first trimester ultrasound examination when SUA

is detected in early gestation (see Chapter 5 for more detail).

Velamentous and Marginal Cord Insertion

The term velamentous umbilical cord describes an insertion of the umbilical cord into

the membranes at the placental margin rather than into the placental surface (Figs. 15.28

a n d 15.29). Velamentous umbilical cord has been reported in about 1% of

pregnancies.56–58 High prevalence of velamentous umbilical cord insertion was found in

spontaneous abortions occurring in 33% of specimens examined between 9 and 12

weeks and in 27% of specimens examined between 13 and 16 weeks of gestation.59

Marginal umbilical cord insertion refers to attachment of the umbilical cord to the

periphery of the placenta (Fig. 15.30) and is noted in 2% to 10% of pregnancies.56–58

Common perinatal complications associated with velamentous and marginal umbilical

cord insertions include miscarriage, prematurity, fetal growth restriction, fetal

malformation, perinatal death, low Apgar scores, and retained placenta.60–63 Several

studies have reported a higher incidence of velamentous cord insertion in pregnancies

of assisted reproduction.64–66 Velamentous cord insertion has a higher prevalence in

multiple gestations. Monochorionic pregnancies with velamentous cord insertion should

be monitored for signs of twin–twin transfusion or selective fetal growth restriction.Figure 15.29: Gray scale (A) and color Doppler (B) of a twin pregnancy at 12

weeks of gestation with velamentous insertion of the umbilical cord into the

dividing membrane (arrow). Velamentous cord insertion is more common in

multiple pregnancies. See text for details.

Figure 15.30: Gray scale (A) and color Doppler (B) of a pregnancy at 11

weeks of gestation with marginal insertion of the umbilical cord. Note that the

umbilical cord inserts on the lateral margins of the placenta rather than centrally

(asterisk).Visualization of the umbilical cord insertion site is feasible in the first trimester and

can be successfully achieved in 93.5% of cases between 9 and 11 weeks of gestation

and in up to 100% of cases at 11 to 14 weeks of gestation, and can be completed within

a 30-second time period.67,68 Assessment of the placental umbilical cord insertion site

should be performed using the appropriate magnification and settings of ultrasound

equipment (Fig. 15.31). It is recommended to identify the free loop of the cord and then

follow it until it reaches the placental surface. Color or power Doppler imaging can

improve visualization of the insertion site (Figs. 15.28 to 15.31) by confirming the

presence of branching vessels. This helps to distinguish true insertion site from an

adjacent free loop of the umbilical cord.

There is limited information on the detection of abnormal placental umbilical cord

insertion by ultrasound in the first trimester. The first case of velamentous cord insertion

diagnosed by transvaginal sonography in the first trimester was published by

Monteagudo et al. in 2000.69 Sepulveda68 reported five cases of velamentous cord

insertions diagnosed in the first trimester and confirmed at term from a group of 533

consecutive singleton pregnancies examined over a 1-year period. Of note, one of the

five pregnancies with velamentous cord insertion was complicated by fetal

chromosomal abnormality (Turner syndrome) and two other women had a history of

infertility, and in one of them, the pregnancy was conceived by intracytoplasmic sperm

injection.68 A study by Hasegawa et al.67 demonstrated that visualization of cord

insertion in the lower third of the uterus between 9 and 13 weeks of gestation was

associated with developmental abnormalities of the placenta and the umbilical cord,

including velamentous and marginal cord insertions, vasa previa, and placenta previa.

Velamentous insertion can be a prerequisite for vasa previa; early prenatal detection of

an abnormal umbilical cord insertion requires follow-up ultrasound at 32 weeks of

gestation looking for the presence of vasa previa.

Figure 15.31: Color Doppler at the cord insertion site in two fetuses at 13 (A)and 12 (B) weeks of gestation. Note the normal central insertion of the

umbilical cord in the placenta. A: Posterior placenta. B: Anterior placenta.

Vasa Previa

Vasa previa refers to the presence of fetal blood vessels between the presenting fetal

parts and the cervix. The fetal blood vessels can run in the fetal membranes unprotected

or the umbilical cord can be tethered to the membranes at the level of the cervical os.

These vessels are prone to compression and bleeding preferentially at the time of

delivery and may cause unexpected fetal death because of hypoxia or exsanguination.

The incidence of vasa previa is approximately 1 in 2,500 deliveries.70 When

undiagnosed, vasa previa has an associated perinatal mortality of 60%, whereas 97 %

of fetuses survive when the diagnosis is made prenatally.71

Ultrasound markers for vasa previa described in the second and third trimester

include resolving low-lying placenta or placenta previa, presence of an accessory

placental lobe (succenturiate lobe), velamentous cord insertion, multiple gestations, or a

suspicion of aberrant vessels crossing over the internal os.72 Pregnancies conceived by

assisted reproduction are also at higher risk for vasa previa.

Information regarding the diagnosis of vasa previa in the first trimester ultrasound is

currently lacking in the literature. Hasegawa et al.73 demonstrated that cases of vasa

previa detected in the second or third trimester of pregnancy occurred only in

pregnancies with cord insertions in the lower third of the uterus between 9 and 13

weeks of gestation. The detection of an abnormal umbilical cord insertion in the first

trimester ultrasound examination should prompt further evaluation for possible vasa

previa in later pregnancy.

Cord Cyst

The diagnosis of a cord cyst in the first trimester is based on the visualization of a

rounded thin-walled anechoic structure within or adjacent to the umbilical cord, which

can be seen from about 8 weeks of gestation (Figs. 15.32 to 15.36). Umbilical cysts can

be single or multiple (Fig. 15.34) and vary significantly in size (Fig. 15.35). The

prevalence of an umbilical cord cyst in the first trimester of pregnancy has been

reported between 0.4% and 3.4%.74,75 Multiple cysts are much less common and affect

about 0.52% of pregnancies in the first trimester.76 Most of the umbilical cord cysts

appear to be transient, with no effect on pregnancy outcome. The majority of them are

incidentally discovered during transvaginal ultrasound between 8 and 9 weeks of

gestation and resolve on follow-up ultrasound at 12 to 14 weeks of gestation or later in

pregnancy. Attention should be paid to differentiate a single umbilical cord cyst from

the yolk sac, which has more echogenic borders and is extraamniotic in location (Figs.

15.32, 15.33, and 15.36).58 Cysts can be found at any location along the length of the

umbilical cord; however, most of them are situated at the middle part of the umbilicalcord.76–78

Figure 15.32: Gray scale ultrasound of a pregnancy at 9 weeks of gestation

with an umbilical cord cyst. Note the location of the umbilical cord cyst close to

the placental insertion of the cord. See the corresponding three-dimensional

ultrasound image in Figure 15.33.Figure 15.33: Three-dimensional ultrasound of the same fetus in Figure 15.32

with a cord cyst at 9 weeks of gestation. The umbilical cord cyst is seen within

the amniotic cavity in contrast to the yolk sac seen outside of the amniotic

cavity.Figure 15.34: Color Doppler ultrasound of a pregnancy at 13 weeks of

gestation with two umbilical cord cysts. Note the varying size of the umbilical

cord cysts with a large (asterisk) and a small (plus sign) cyst. Color Doppler

shows the two umbilical arteries within the cord.

Figure 15.35: Color Doppler ultrasound of a pregnancy at 13 weeks of

gestation with an umbilical cord cyst. Note that the umbilical vessels surround

the cyst on color Doppler.1.

2.

3.

4.

5.

Figure 15.36: Gray scale ultrasound of a fetus at 8 weeks of gestation with an

umbilical cord cyst (cord cyst). Note that the umbilical cord cyst is seen within

the amniotic cavity in contrast to the yolk sac seen outside of the amniotic

cavity. Compare with Figures 15.32 and 15.33.

Initial studies established an association between umbilical cord cysts found in the

first trimester and a higher incidence of aneuploidy, congenital abnormalities, and

overall poor pregnancy outcomes.74 More recent large series of first trimester umbilical

cord cysts did not corroborate the same associations with poor pregnancy outcome.77

Based on the evidence provided by recent literature, the first trimester umbilical cord

cyst should not be considered an independent marker of poor pregnancy outcome

regardless of its location, size, and number, especially when the cord cysts resolve on

follow-up ultrasound ex