CHAPTER 4 • Fetal Biometry and Pregnancy Dating in the First Trimester. First Trimester Ultr

 CHAPTER 4 • Fetal Biometry and Pregnancy Dating in the First Trimester

INTRODUCTION

Accurate performance of an ultrasound examination in the first trimester is important

given its ability to confirm an intrauterine gestation, assess number and viability of

embryos or fetuses, accurately date the pregnancy, and diagnose major fetal

malformations. In this chapter, we present the approach and indications to the first

trimester ultrasound examination, the parameters of pregnancy dating, and the ultrasound

markers of pregnancy failure. Normal fetal anatomy and fetal malformations will be

discussed in detail in subsequent chapters.

APPROACH TO THE FIRST TRIMESTER

ULTRASOUND EXAMINATION

The first trimester ultrasound examination can be performed transabdominally or

transvaginally. There is general consensus that, with rare exceptions, obstetrical

ultrasound examinations at less than 10 weeks of gestation are best performed

transvaginally. The transvaginal approach provides for higher resolution than the

transabdominal approach and positions the transducer in close proximity to the target

anatomic region (gestational sac). Beyond the 12th week of gestation, the

transabdominal approach with high-resolution transducers and with optimal imaging can

provide sufficient details to allow for systemic assessment of fetal anatomy. In the

presence of first trimester suspected fetal malformations, a combined transabdominal

and transvaginal approach is recommended.

INDICATIONS FOR THE FIRST TRIMESTER

ULTRASOUND EXAMINATION

There is currently varying opinions on whether the first trimester ultrasound examination

is offered routinely to all pregnant women or is indication driven.1,2 Indications for the

first trimester ultrasound are many and most pregnant women in resourced settings••••••••••

receive at least one such ultrasound during their pregnancies. With mounting evidence

on the important role of the first trimester ultrasound in pregnancy dating, aneuploidy

risk assessment, diagnosis of multiple pregnancies, and in detection of major fetal

malformations, the authors believe that ultrasound in early gestation will ultimately be

offered routinely to pregnant women. Table 4.1 lists common indications for the first

trimester ultrasound examination in pregnancy.

SONOGRAPHIC LANDMARKS IN THE FIRST

TRIMESTER

The normal intrauterine pregnancy undergoes significant and rapid change in the first

trimester, from a collection of undifferentiated cells to a fetus within an amniotic sac

connected to a placenta and a yolk sac. This significant progression can be seen on

ultrasound beginning with the chorionic sac, which is the first sonographic evidence of

pregnancy and progressing to the embryo and fetus with cardiac activity. Identifying

ultrasound landmarks and understanding its normal progression in the first trimester help

in confirming a normal pregnancy and in the diagnosis of pregnancy failure.

Table 4.1 • Common Indications for Ultrasound examination in the First

Trimester

Amenorrhea

Pelvic pain

Vaginal bleeding

Unknown menstrual dates

Subjective feeling of pregnancy

Uterus greater or smaller than dates on clinical evaluation

Positive pregnancy test

Aneuploidy risk assessment and nuchal translucency measurement

Fetal anatomy survey

Ruling out multiple pregnancies

Gestational Sac

The gestational sac, also referred to as the chorionic cavity, is the first sonographic

evidence of pregnancy. The gestational sac on transvaginal ultrasound is first seen at 4

to 4.5 weeks from the first day of the last menstrual period (LMP) (Fig. 4.1). When the

gestational sac has a mean diameter of 2 to 4 mm, its borders appear echogenic, which

makes its demonstration easy (Fig. 4.1). The echogenic ring of the gestational sac is an

important ultrasound sign, which helps to differentiate it from intrauterine fluid or blood

collection (Fig. 4.2). The shape of the gestational sac is first circular but with the

appearance of the yolk sac and the embryo it becomes more ellipsoid (Fig. 4.3).Yolk Sac

The yolk sac is seen at 5 weeks of gestation (menstrual age) on transvaginal ultrasound,

as a small ring within the gestational sac with highly echogenic borders (Figs. 4.3 and

4.4). It has a diameter of around 2 mm at 6 weeks and increases slowly to around 6 mm

at 12 weeks. The first detection of the embryo by ultrasound is noted in close proximity

to the free wall of the yolk sac, because the yolk sac is connected to the embryo by the

vitelline duct. A small yolk sac with a diameter less than 3 mm between 6 and 10 weeks

or a diameter of more than 7 mm before 9 weeks is a cause for concern for an abnormal

pregnancy and thus this observation requires a follow-up ultrasound examination to

assess normalcy of pregnancy (Fig. 4.5A and B).

Figure 4.1: Midsagittal plane of a uterus with a gestational sac at 4.5 weeks of

gestation. Note the echogenic borders (arrows) of the gestational sac. The

echogenic borders (ring) of the gestational sac help to differentiate it from an

intrauterine fluid or blood collection. The fundus of the uterus is labeled for

orientation.Figure 4.2: Midsagittal (A) and transverse (B) planes of two uteri showing fluid

accumulation (asterisk) between the decidual layers. This finding should not be

confused with an intrauterine gestational sac. See text for details.

Amnion

The amniotic sac develops as a thin echogenic structure surrounding the embryo (Fig.

4.6). The amniotic sac appears following the appearance of the yolk sac and just before

the appearance of the embryo. Whereas the gestational sac shows variations in size and

shape, the growth of the amniotic sac is closely related to that of the embryo between 6and 10 weeks of gestation.

Embryo

The embryo is first seen on transvaginal ultrasound as a focal thickening on top of the

yolk sac, at around the fifth menstrual week (Fig. 4.7). First cardiac activity is typically

seen by 6 to 6.5 weeks. The embryo can be recognized by high-resolution transvaginal

ultrasound at the 2 to 3 mm length size (Fig. 4.7), but cardiac activity can be consistently

seen when the embryo reaches a 5 to 7 mm in length or greater. Embryonic heart rate

increases rapidly in early gestation being around 100 to 115 before 6 weeks, rising to

145 to 170 at 8 weeks, and dropping down to a plateau of 137 to 144 after 9 weeks of

gestation. The size of the embryo increases rapidly by approximately 1 mm per day in

length. Note that the embryo develops within the amniotic cavity and is referred to as

intraamniotic whereas the yolk sac is outside of the amniotic cavity and is referred to as

extraamniotic. The fluid that the yolk sac in embedded within is the extraembryonic

coelom.

Figure 4.3: Midsagittal plane of a uterus with a gestational sac at 6 weeks of

gestation. Note the presence of a yolk sac and a small embryo. The shape of

the gestational sac is more ellipsoid than circular.Figure 4.4: Midsagittal plane of a uterus with a gestational sac at 5.5 weeks of

gestation. Note the yolk sac seen within the gestational sac with highly

echogenic borders.

The appearance of the embryo on ultrasound changes from 6 to 12 weeks of

gestation. At 6 weeks of gestation, the embryo appears as a thin cylinder with no

discernible body parts, “the grain of rice appearance” (Fig. 4.8). As gestational age

advances, the embryo develops body curvature and clear delineation on ultrasound of a

head, chest, abdomen, and extremities, “the gummy-bear appearance” (Figs. 4.9 and

4.10). Clear delineation of a head, chest, abdomen, and extremities on gray scale

ultrasound is noted at 10 weeks of gestation and beyond (Fig. 4.11). Close observation

of anatomic details on transvaginal ultrasound at or beyond 12 weeks of gestation may

allow for the diagnosis of major fetal malformations. This will be discussed in detail in

Chapters 8 to 14, organized by anatomic organ system.

FIRST TRIMESTER PREGNANCY DATING

One of the most important aspects of the first trimester obstetric ultrasound is pregnancy

dating as this is accomplished by performing few simple biometric measurements: (1)

the gestational sac diameter, when no embryo is seen; (2) the length of the embryo, or

crown-rump length (CRL); (3) in the late first trimester (12 to 14 weeks), the biparietal

diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur

length (FL). Obtained biometric values are compared to established reference ranges to

provide for accurate dating. With an accurate ultrasound-derived gestational age in the

first and second trimesters of pregnancy, ultrasound can reliably date a pregnancy with

unknown dates and establish an estimated date of delivery with accuracy.Figure 4.5: A and B: Two gestational sacs with abnormal size of yolk sacs:

small in A and large in B. Abnormal size of yolk sacs is a concern for an

abnormal pregnancy and follow-up ultrasound is recommended.

Figure 4.6: Gestational sac at 7 weeks of gestation. The amniotic sac is seen

as a thin reflective circular membrane. The yolk sac and vitelline duct are seen

as extraamniotic structures.Figure 4.7: Transvaginal ultrasound of a gestational sac with an embryo

measuring 1.8 mm in size. Note the proximal location of the yolk sac to the

embryo.

Figure 4.8: Gestational sac at 6 weeks with an embryo measuring 5.1 mm in

crown-rump length (CRL). Note the straight shape of the embryo, resembling a

“grain of rice.” The yolk sac is seen adjacent to the embryo. GA, gestational

age.Figure 4.9: Gestational sac with an embryo at 8 weeks. Note the body

curvature of the embryo, resembling a “gummy bear” in shape. The yolk sac is

seen adjacent to the embryo.

Figure 4.10: Three-dimensional ultrasound in surface mode of a fetus at 8

weeks of gestation showing the body curvature of the embryo, resembling a“ •

gummy bear” in shape. The yolk sac is seen adjacent to the embryo and the

vitelline duct is seen connecting the yolk sac to the umbilical cord.

In clinical medicine, the age of an embryo or a fetus is expressed in weeks of

gestation and not in months, and these weeks are calculated from the first day of the

LMP, which corresponds to two additional weeks from the date of conception.

Gestational age is therefore calculated from the first day of the LMP and roughly

corresponds to the dates of conception plus about 14 days. Ultrasound equipment has an

integrated calculator, which calculates the estimated date of delivery as the LMP is

entered. Formulas for calculating gestational age from various biometric measurements

are also part of the software of ultrasound equipment.

Figure 4.11: Gestational sac with an embryo at 10 weeks of gestation. Note

the clear delineation of a head, chest, abdomen, and extremities. CRL, crownrump length.

In estimating gestational age by ultrasound, it is important to remember these critical

points:

Once an established date of delivery is assigned to a pregnancy following an

ultrasound examination, irrespective of whether the assigned established dates were

those by ultrasound or by menstrual dates, these dates should not be changed later on

in pregnancy.• • •

If a patient reports no menstrual dates, ultrasound in the first or second trimester

should date the pregnancy and establish the estimated date of delivery.

If the ultrasound biometric measurements vary from the menstrual dates by more than 5

to 7 days in the first trimester, then ultrasound should be used to establish the date of

delivery.3

Ultrasound dating of pregnancy is most accurate in the first trimester.

BIOMETRIC MEASUREMENTS IN THE FIRST

TRIMESTER

Biometric measurements for first trimester dating include the mean gestational sac

diameter (MSD), the CRL and the fetal BPD and HC (greater than 11 weeks). The most

accurate and reproducible biometric measurement is the CRL and should be the

preferred biometric measurement for pregnancy dating when feasible.

Mean Sac Diameter

Because the gestational sac is the first evidence of pregnancy on ultrasound and is first

visualized within the endometrial cavity at 4 to 4.5 weeks after the LMP, its detection

and measurement can be used to confirm and date a pregnancy. The biometric

measurement for pregnancy dating uses the MSD, calculated as the arithmetic mean

diameters derived from its greatest sagittal, transverse, and coronal planes (Fig. 4.12A

and B). The presence of a gestational sac in the endometrial cavity confirms the

presence of an intrauterine pregnancy but not the viability of the embryo. The presence

of a gestational sac within the endometrial cavity without an embryo, suggests that the

pregnancy is at 5 to 6 weeks of gestation. It is not recommended to rely solely on the

MSD for estimating the due date, as the CRL is a more precise dating method and

should be the preferred choice.

Crown-Rump Length

The CRL corresponds to the length of the embryo in millimeters. Although the name

implies a measurement from the crown to the rump of the embryo, the actual

measurement corresponds to the longest “straight line” distance from the top of the head

to the rump of the embryo/fetus (Figs. 4.7, 4.8, 4.11, and 4.13), despite the noted body

curvature. The CRL measurements are more accurate in the embryo/fetal neutral

position and between 11+0 and 13+6 weeks of gestation. When measuring the CRL, the

operator should use the mean of three discrete measurements, obtained in a midsagittal

plane of the embryo/fetus. It is recommended to follow these parameters when dating a

pregnancy in the first trimester (<14 weeks) by CRL:F • •

igure 4.12: A and B: Measurements of the mean sac diameter (MSD) of a

gestational sac at 5 weeks. The MSD is calculated as the arithmetic mean

diameters derived from its greatest sagittal (1), transverse (2) in A and coronal

planes (1) in B.

Figure 4.13: Crown-rump length (CRL) measurement of a fetus at 12 + 5

weeks of gestation. Note that the CRL measurement corresponds to the

longest straight line from the top of the head to the rump region. GA,

gestational age.

For pregnancies at less than 9 weeks of gestation, a discrepancy of more than 5 days

from LMP is an appropriate reason for changing the expected date of delivery

(EDD).3

For pregnancies between 9 and 13 6/7 weeks of gestation, a discrepancy of more than7 days should result in a change in the EDD.3

The CRL increases rapidly at a rate of approximately 1.1 mm per day. An

approximate formula to calculate gestational age from the CRL is gestational age in days

= CRL (mm) + 42; however, this may not be needed because most ultrasound equipment

have integrated software, which allows gestational age determination upon measurement

of CRL or other biometric data.

Figure 4.14: Biparietal diameter (BPD) measurement of a fetus at 13 weeks of

gestation. According to the setting used, the measurement is achieved either

outside to outside (A) or outside to inside (B). See Table 4.2 for details.

Biparietal Diameter

Measurement of the BPD, HC, AC, and FL in the first trimester is typically performed at

12 to 14 weeks and follows the same anatomic landmarks as those in the second and

third trimesters. The BPD is measured in an axial plane of the fetal head at the level of

the thalami (Fig. 4.14). Sonographic landmarks identifying the correct BPD plane are

listed in Table 4.2. In some settings, the BPD is measured by placing the near and far

calipers on the outside of the proximal and distal parietal bones (Fig. 4.14A) and in

other settings, the near caliper is placed on the outside of the parietal bone and the far

caliper is placed on the inside of the parietal bone (Fig. 4.14B). Readers should

conform to their regional standards for BPD measurements.

Head Circumference•••••••••••••

The HC is measured in the trans-thalamic axial view, which is the same plane as that for

the BPD measurement (Fig . 4.15). We recommend that you perform the HC

measurement following the BPD measurement. This approach allows the operator to

utilize the calipers placed for BPD measurement, which expedites the process. It is of

note that when the HC is being measured, the lower caliper from the BPD diameter

should be moved to the outer bony parietal edge (Fig. 4.15).

Table 4.2 • Sonographic Landmarks for the Measurement of the Biparietal

Diameter (BPD) Plane

Focal zone at appropriate level

Image magnified

Axial plane of the fetal head

Symmetric appearance of cerebral hemispheres

Midline falx imaged

Thalami imaged

Cavum septi pellucidi imageda

Insula imageda

No cerebellum visualized

Near caliper on outside edge of bone

Far caliper on inside/outside edge of bone (see text)

Measurement at widest diameter

Measurement perpendicular to falx

aNot visible in the first trimester•

Figure 4.15: Head circumference (HC) measurement of a fetus at 13 + 5

weeks of gestation. Note that the calipers are placed outside to outside for HC

measurement. GA, gestational age.

Abdominal Circumference

The AC is measured on a transverse (axial) section of the upper fetal abdomen.

Sonographic landmarks identifying the correct plane for the AC measurement are listed

in Table 4.3 and Figure 4.16.

Femur Length

In order to optimize the measurement of the FL, the whole femur diaphysis should be

displayed on the screen, and the angle between the insonating beam and the shaft of the

femur should be kept in the range of 45 to 90 degrees in order to avoid underestimating

the length of the femur due to ultrasound wave deflection (Fig. 4.17). The longest

visible diaphysis should be measured by placing each caliper at the end of the

diaphysis. Femur measurements can be difficult to perform in early gestation, as the

diaphyseal segment of the bone is not fully ossified. Sonographic landmarks identifying

the correct plane for the FL measurement are listed in Table 4.4.

Table 4.3 • Sonographic Landmarks for the Measurement of the

Abdominal Circumference (AC) Plane

Focal zone at appropriate level••••••••••

Image magnified

Axial plane of the abdomen

Abdomen as circular as possible

Spine imaged in cross section in 3- or 9-o’clock position if possible

Stomach bubble imaged

Intrahepatic portion of the umbilical vein imaged in a short segmenta

No more than one rib visible on either side laterally

Kidneys not to be visualized in the image

Surrounding skin seen in its entirety if possiblea

Measurement of circumference ellipse on outside edge of skina

aNot clearly visible in the first trimester

Figure 4.16: Abdominal circumference (AC) measurement of a fetus at 13

weeks of gestation. See Table 4.3 for details. GA, gestational age.••••••

Figure 4.17: Femur length (FL) measurement of a fetus at 13 weeks of

gestation. See Table 4.4 for details. GA, gestational age.

Table 4.4 • Sonographic Landmarks for the Measurement of the Femur

Length (FL)

Focal zone at appropriate level

Image magnified

Whole femur diaphysis imaged

Ultrasound beam perpendicular to long axis of femur

Calipers placed at end of diaphysis

Longest visible diaphysis is measured

ELEMENTS OF PREGNANCY FAILURE

Pregnancy failure can occur in up to 10% to 15% of pregnancies. Suspected pregnancy

failure is thus a common indication for ultrasound examination in the first trimester. The

diagnosis can often be made by ultrasound, typically before symptoms develop by

patients. Depending on the gestational age of pregnancy, several scenarios can be• • •• ••

• •• •

expected:

Pregnancy confirmed by a positive pregnancy test but no gestational sac is noted in the

uterine cavity by ultrasound, suggesting the differential diagnosis of an incomplete

miscarriage, an ectopic pregnancy, or an early intrauterine pregnancy that is not yet

recognizable by transvaginal ultrasound.

Gestational sac noted by transvaginal ultrasound, but no signs of embryo or yolk sac

within it.

An embryo visualized on transvaginal ultrasound, but no cardiac activity detected.

An embryo with cardiac activity detected, but various measurements are out of range

(heart rate, size of yolk sac, embryo, amniotic sac, etc.).

Presence of subchorionic bleeding, with or without clinical signs of bleeding.

Abnormal anatomic appearance of the embryo.

In many conditions, if the health of the patient is not in danger (bleeding, pain etc.)

and an ectopic pregnancy is not in the differential diagnosis, a follow-up ultrasound

examination is helpful to assess for change in the ultrasound findings and in confirming

the suspected diagnosis. Given that the developing gestational sac undergoes notable

significant change on a weekly basis in the first trimester, follow-up ultrasound that fails

to show a noticeable change after 1 week or more casts a poor prognostic sign and can

confirm the diagnosis of a suspected failed pregnancy. The presence of subchorionic

bleeding is generally associated with a good outcome in the absence of other markers of

pregnancy failure (see Chapter 15). It is the opinion of the authors that in the absence of

specific findings of failed pregnancy, conservative management with follow-up

ultrasound examination is helpful in the evaluation of a suspected failed pregnancy in

early gestation. Table 4.5 lists specific findings of failed pregnancy in the first

trimester, which when noted can establish the diagnosis without a need for a follow-up

examination.4

Table 4.5 • Diagnostic Signs of Early Pregnancy Failure in the First

Trimester

Crown-rump length of equal to or greater than 7 mm without cardiac

activity

Mean sac diameter of equal to or greater than 25 mm without an embryo

Absence of embryo with heartbeat at two or more weeks after an

ultrasound that showed a gestational sac without a yolk sac

Absence of embryo with heartbeat at 11 days or more after an ultrasound

that showed a gestational sac with a yolk sac

CONCLUSION

The first trimester ultrasound examination is an important step in the evaluation of the1.

2.

3.

4.

pregnancy as it allows for confirmation of an intrauterine gestation, accurate pregnancy

dating, and evaluation of fetal anatomy. It is of note that significant change occurs in the

first trimester and this change can be detected by transvaginal ultrasound examination.

Sequential steps of the normal development of the pregnancy should be known in order

to better compare the actual ultrasound findings with the corresponding gestational age.

This is the basic knowledge that is needed in order to differentiate a normal from an

abnormal gestation. Following chapters in this book provide detailed evaluation for

screening and diagnosis of major fetal malformations in the first trimester of pregnancy.