Case 20 A 35-year-old woman booking for
antenatal care
Mrs Todd is a 35-year-old woman in her third pregnancy.
Her last menstrual period (LMP) was 12 weeks ago and she
has been well in this pregnancy up until now. She is
attending for her first antenatal visit.
Antenatal screening is designed to identify those pregnancies at greater risk of complications. It comprises
information from the history, as well as ultrasound scanning, blood tests and other investigations. As a result of
cost and clinical effectiveness, only screening tests recommended by the National Institute for Health and Clinical
Excellence (NICE) are included here, although other
screening tests are indicated during some pregnancies.
What information do you need from
the history?
1 Past obstetric history
Number and outcomes of previous pregnancies
Any problems with the pregnancies including:
pre - eclampsia, abruption, etc.
mode of delivery
gestation at delivery
birth weights
any postnatal problems
2 Past medical history
3 Drug history, including allergies
4 Social history
Ethnicity
Occupation
Social support
Smoking
Alcohol intake
Illicit drug use
5 Family history, including partner and his family
She tells you that her first two pregnancies were
uncomplicated vaginal deliveries at term of normal-sized
babies. She has no significant past medical history or
allergies and does not take any medicines. She is a legal
secretary, does not drink alcohol or use illicit drugs but
smokes 30 cigarettes per day. She is adopted and unable to
provide any information regarding family history but there
are no problems on her husband’s side.
What advice will you offer at
this stage?
She should try to cut down or stop smoking. If her
partner smokes they could stop together. Smoking cessation advice, support groups and nicotine replacement
therapy are all available in pregnancy via referral from
maternity services or her GP.
She accepts your offer to refer her for smoking cessation
advice as she realizes it is detrimental to hers and her baby’s
health.
She has read the leaflet that the hospital sent about
screening in pregnancy but is very unclear and would like
you to explain it to her.
What would you discuss with her?
Screening in pregnancy is designed to detect conditions
that have no symptoms but can have detrimental effects
on her or her baby. They usually comprise blood tests,
urine tests and ultrasound scanning. They aim to detect
infections, fetal abnormalities and abnormalities in the
maternal blood (Boxes 20.1 & 20.2 ). Screening is also
available for some medical conditions that can affect the
pregnancy.
Mrs Todd accepts haematological screening and screening
for infection. She tells you she is very worried about the risk
of Down’s syndrome in this pregnancy as her neighbour had
a baby with Down’s syndrome after low risk screening. She
asks you if she can have an ultrasound scan to make sure
her baby does not have Down’s syndrome.
Obstetrics and Gynaecology: Clinical Cases Uncovered.
By M. Cruickshank and A. Shetty. Published 2009 by Blackwell
Publishing. ISBN 978-1-4051-8671-1.Case 20 133
PART 2: CASES
What will you tell her?
Ultrasound is a useful test for many fetal abnormalities
but is not a good test for Down ’ s syndrome as many
babies with Down ’ s syndrome appear normal on scan.
Screening tests are available for Down ’ s syndrome and
other chromosomal abnormalities but only provide a risk
of the pregnancy being affected. That means that low risk
results do not definitely mean the baby is unaffected
(false negatives). Conversely, a high risk result does not
definitely mean the baby is affected (false positives). The
simplest test of risk is based on maternal age. At 20 years
the risk is 1 in 1440 rising to 1 in 32 aged 45. Mrs Todd ’ s
age related risk at 35 years is 1 in 338. Other screening
tests involve combinations of blood tests, ultrasound
scans and some information from the mother ’ s history.
If a screening test has a high - risk result (usually >1 in
250), because the baby is not definitely affected the
parents would be offered a diagnostic test (amniocentesis
or chorionic villous sampling [CVS]) to provide a definitive diagnosis.
Mrs Todd understands what you have told her and does not
think she wants to have an amniocentesis straight away but
would consider it if she was uncomfortable with the results
of the screening tests. She asks you for more information
about the available tests.
Box 20.1 Screening for haematological conditions
Anaemia
Adequate haemoglobin concentrations prevent symptoms
and reduce the need for postnatal blood transfusion. The
most common cause of anaemia in pregnancy is iron
deficiency. Screening for anaemia should be offered at
booking and 28 weeks. This allows time for treatment if
anaemia is detected.
Hb levels <11g/dL at booking or <10.5g/dL at 28 weeks
should be investigated with a serum ferritin. Levels <30μg/L
indicate inadequate iron stores and supplementation should
be prescribed.
Blood grouping and red cell alloantibodies
Identifying blood group, rhesus D status and red cell
antibodies is important in preventing haemolytic disease of
the newborn and to identify potential transfusion reactions.
Identifying rhesus D-negative women also allows them to
be offered appropriate antenatal and postnatal
immunoprophylaxis to try to prevent rhesus D
alloimmunization in subsequent pregnancies.
Current recommendations are testing of ABO, rhesus D
typing and screening for irregular antibodies at booking
and 28 weeks’ gestation.
Sickle cell disorders and thalassaemia
Haemoglobin disorders are autosomal recessive but it is
possible to inherit more than one. They can cause a
variety of illnesses of varying severity and the aim of
screening is to identify women at risk in early pregnancy
so that genetic counselling can be provided and they can
make a decision about invasive diagnostic tests and
continuation of their pregnancy.
Screening can be based on a combination of an
ethnicity question to identify women at higher risk, then
investigation of the high risk group for haemoglobin
abnormalities, or on offering laboratory screening to all
pregnant women.
It is important to discuss with her the methods,
timing and possible consequences of screening, including the possibility of being faced with a decision
about termination of an affected pregnancy. You should
also discuss the options available for screening in
your hospital. Screening for Down ’ s syndrome includes
screening the pregnancy for some other chromosomal
abnormalities (e.g. trisomy 13 and 18; Boxes 20.3 & 20.4 ;
Fig. 20.1 ). You should tell her that she can change
her mind and opt out of the screening programme at
any time.
She is happy to accept the combined test and has her
booking scan and NT performed. She has a 12-week
singleton continuing pregnancy with neural tubes within the
normal range. The midwife performs her booking bloods
along with the pregnancy-associated plasma protein A
(PAPP-A) and human chorionic gonadotrophin (hCG) as part
of the combined test (Box 20.5).
At 20 weeks she returns to the hospital for her detailed
ultrasound scan. The combined test gave the pregnancy a
risk of Down’s syndrome of 1 in 1775 and Mrs Todd and
her partner are reassured by this. They wonder if they still
need the detailed scan.134 Part 2: Cases
PART 2: CASES
Box 20.2 Screening for infections
Asymptomatic bacteriuria
Persistent bacterial colonization of the urinary tract occurs in
2–5% of pregnant women in the UK. Pregnant women with
asymptomatic bacteruria have an increased risk of
pyelonephritis and preterm birth. Treatment with antibiotics
reduces these risks.
It is therefore recommended that all pregnant women be
screened for this with mid-stream urine culture early in
pregnancy and treated with appropriate antibiotics if it is
detected.
Hepatitis B virus infection
Hepatitis B infection is often asymptomatic and may be
highly infectious. Mother–child transmission results in a
significant proportion of chronic carriage in the baby and
chronic carriage results in a high risk of hepatocellular
carcinoma and cirrhosis. Approximately 95% of mother–
child transmission is preventable through the administration
of vaccine and immunoglobulin to the baby at birth.
Therefore, all pregnant women should be offered serum
screening for hepatitis B infection to allow effective postnatal
intervention to be offered. Immunoglobulin administration
and hepatits B vaccination for babies of infected women
reduces the risk of mother–child transmission.
HIV
The prevalence of HIV in women of reproductive age is
increasing in the UK. In the absence of intervention,
mother–child transmission occurs in around 25% of
deliveries. Appropriate interventions (see Case 26) can
reduce this figure to <1%.
All pregnant women should be offered screening for HIV
antibodies early in pregnancy so appropriate interventions
can be put in place to reduce vertical transmission.
Rubella
Rubella infection in early pregnancy can have catastrophic
consequences. There is no treatment to prevent or reduce
mother–child transmission of rubella for the current
pregnancy. Screening for rubella antibodies enables
susceptible women to be vaccinated in the postnatal
period for the protection of future pregnancies.
Syphilis
Syphilis is rare now in the indigenous UK population but
may be more common in the increasing immigrant
population.
In pregnant women with early untreated syphilis,
70–100% of babies will be infected and one-third will be
stillborn. Mother–child transmission is associated with
neonatal death, congenital syphilis (which may cause
long-term disability), stillbirth and prematurity.
Early detection and parenteral treatment of the mother
with penicillin effectively prevents mother–child
transmission. The outcome of treated pregnancies with
syphilis is comparable with the outcome of pregnancies in
women who never had syphilis. Screening also allows for
appropriate referral to genitourinary medicine (GUM)
services and treatment of the mother. Therefore, despite
the rarity of the condition as a result of the efficacy of
available treatment and serious consequences of
non-treatment, screening continues.
Box 20.3 Screening for Down’s syndrome (trisomy 21) and other common chromosomal abnormalities
Antenatal screening can take place in the first or second
trimester. It is recommended that the test performed should
have a detection rate above 75% for a false positive rate
less than 3% (NICE guideline on antenatal screening; see
further reading). The tests all have advantages and
disadvantages in terms of timing and false positive and
negative rates but meet the above criteria as a minimum.
11–14 weeks – the combined test
Nuchal translucency (NT) measures the subcutaneous space
between skin and cervical spine in early pregnancy. Increased
measurements are associated with increased risk (Fig. 20.1).
This measurement is used in combination with maternal
serum levels of human chorionic gonadotrophin (hCG) and
pregnancy-associated plasma protein A (PAPP-A). The NT
can usually be performed with the booking scan.
14–20 weeks – the quadruple test
Maternal serum is sent for the quadruple test that
comprises hCG, alpha-fetoprotein (AFP), unconjugated
estriol (uE3) and inhibin A.
11–14 weeks and 14–20 weeks
The integrated test – NT, PAPP-A + hCG, AFP, uE3,
inhibin A
plus
the serum integrated test – PAPP-A + hCG, AFP, uE3,
inhibin ACase 20 135
PART 2: CASES
What would you discuss with her
regarding the mid-trimester
anomaly scan?
The screening test she has already had was for the
common trisomies. The detailed ultrasound aims to
detect structural abnormalities in the baby. However,
detailed ultrasound scans do not detect all abnormalities.
If any abnormalities are detected during this screening
scan a more detailed scan will be offered. If the baby has
a major abnormality it may mean having to reach a decision about whether or not to continue with the pregnancy. It can also give valuable information regarding a
problem that may require specialist paediatric services,
and may help with deciding the place of birth of a baby
with a known abnormality (Box 20.6 ; Fig. 20.4 ; Table
20.1 ).
Mrs Todd is happy to have her detailed ultrasound scan
which shows that her baby appears structurally normal. She
is very reassured and able to relax through the latter half of
her pregnancy.
Box 20.4 Screening for neural tube defects
A maternal serum AFP carried out between 15 and 21
weeks can indicate a high risk for neural tube defects
(NTDs). A value >2mean of median is considered high risk
and these women would be offered detailed ultrasound
scans to look for a cause for the elevated AFP.
Problems other than NTDs that may cause an elevated
AFP:
• Inaccurate pregnancy dating
• Bleeding in early pregnancy
• Multiple pregnancy
• Fetal death
• Abdominal wall defects – gastroschisis or exomphalos
Sometimes an elevated AFP is unexplained and these
pregnancies remain at increased risk of intrauterine
growth restriction, preterm delivery and pre-eclampsia,
closer surveillance of these pregnancies may be
warranted.
If routine detailed ultrasound scanning is offered to all
women at 20 weeks’ gestation, most of the abnormalities
resulting in an elevated AFP would be detected and it may
be unnecessary to offer this test in addition to a routine
detailed ultrasound scan.
Figure 20.1 Nuchal thickness on scan.
Box 20.5 Diagnostic tests
If a pregnancy is found to be at high risk of a
chromosomal abnormality the woman then has the option
of an invasive diagnostic test to identify the karyotype of
the fetus.
Chorionic villous sampling (CVS) can be performed
between 10 and 14 weeks’ gestation. It takes a tiny
amount of tissue from the placenta and carries a 2% risk
of miscarriage (Fig. 20.2).
Amniocentesis can be performed from 15 weeks’
gestation. Amniotic fluid is withdrawn from around the
fetus and the fetal fibroblasts in the fluid are cultured to
give the fetal karyotype. It carries approximnately 1% risk
of miscarriage (Fig. 20.3). The provisional result is usually
available after 24–72 hours (from quantitative fluorescence
polymerase chain reaction [QF-PCR]) with a full karyotype
in a few weeks.
In addition to the risk of miscarriage, there is an
approximtely 1% risk of mosaicism, culture failure and
requiring repeat testing depending on which test is
undertaken.136 Part 2: Cases
PART 2: CASES
Figure 20.2 Chorionic villus sampling.
Chorionic villi
Figure 20.3 Amniocentesis.
Probe
Placenta
Womb
(uterus)
Amniotic
fluid
Entrance of womb (cervix) Rectum
Vagina
Bladder
Needle
Figure 20.4 Gastroschisis.
Box 20.6 Detailed ultrasound scan
Many abnormalities can be detected on mid-trimester
ultrasound:
• Cleft lip
• Ventricular and atrial septal defects
• Diaphragmatic herniae
• Absent or abnormal kidneys
• Gastroschisis and exomphalos (Fig. 20.4)
• Spina bifida
• Limb abnormalities
The detection rates vary with the:
• Type of abnormality being screened
• Gestational age at scanning
• Skill of the operator
• Quality of the equipment
• Time allotted for the scanCase 20 137
PART 2: CASES
Table 20.1 Percentage of fetal abnormalities detected by
routine ultrasound screening in the second trimester (evidence
level IIa). As detection rates vary, all departments providing
detailed scanning services should monitor their own results
and provide local detection rates to assist patient’s decisionmaking about detailed ultrasound scan.
Anatomical system Detected (%)
Central nervous system 76
Urinary tract 67
Pulmonary 50
Gastrointestinal 42
Skeletal 24
Cardiac 17
Further reading
Antenatal care: routine care for the healthy pregnant woman.
NICE Clinical Guideline CG62, March 2008 .
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