Antenatal Screening
Introduction
Modern antenatal care is based on
the assessment of risk and identification of the most appropriate care pathway
for a pregnant woman. Obstetric ultrasound is a routine tool in antenatal
screening for detecting foetal anomalies. Low risk women are offered ultrasound
screening in the first and second trimesters, and as these are anomaly scans
appropriate care and counselling should be immediately available.
Primary care practitioners will
refer a pregnant woman to ante- natal care and aim for a first appointment with
ultrasound scanning at approximately 10 weeks into the pregnancy from the date
of the last menstrual period (8 weeks after fertilisation). A full obstetric
history should be taken and it is good practice to take full gynaecological and
medical histories. At the booking appointment the midwife will initiate a close
relationship with the mother, and for primigravida women this is the
opportunity to discuss the effects of early pregnancy and non‐specific
symptoms. Tiredness, nausea and vomiting may be worrying, but hyperemesis
gravidarum (excessive nausea and vomiting) should be identified and treated.
The mother will be weighed at this meeting, but normally weight is not
monitored throughout pregnancy.
The first‐trimester ultrasound scan,
sometimes referred to as the ‘dating scan’, is performed at a minimum of 10
weeks’ gestation and usually before 14 weeks. Scanning at this stage will
confirm foetal viability, give gestational dating information, identify
multiple pregnancy, define chorionicity (see Chapter 11) and look for
indicators of anomalous development (Figure 48.1). These indicators include
nuchal translucency, abdominal wall defects (see Chapter 35) and brain
anomalies (see Chapter 44). Nuchal translucency screening is not reliable in
smaller foetuses and other anomalies may also be missed. Nuchal translucency
measures the thickness of fluid between the cervical spine and skin, and is
associated with a number of chromosomal anomalies such as Down syndrome, Turner
syndrome, trisomy 18 and trisomy 13, and with cardiac anomalies (Figure 48.2).
Skeletal dysplasias may be detectable by ultrasound in the first trimester, and
in the near future cardiac defects may be screened for as the resolution of
ultrasound scanning improves.
The most reliable way to ‘date’
the embryo at this stage is by measuring the foetal crown‐to‐rump length (CRL).
If the first ultrasound scan is not performed until later in pregnancy, the
biparietal diameter and head circumference will provide a more accurate indication
of gestational timescale. If the dating scan is not performed until the third
trimester, a measurement of the femur together with either the biparietal
diameter or head circumference will increase dating accuracy (Figure 48.3).
Based on a number of criteria a
mother is assessed as being either ‘low risk’, with a high probability of a
straightforward delivery and a healthy baby, or as a mother who will need a
higher level of antenatal care. In either case after assessment a ‘low risk’ mother’s
care parameters may change depending upon her health and symptoms that may
develop later in gestation.
Basic antenatal tests are
performed from first contact and confirmation of pregnancy at 8–12 weeks to
book the mother into the antenatal care system with the first ultrasound dating
scan, followed by a care appointment at 16 weeks and a second ultrasound scan
at around 20 weeks. Urine is periodically tested for albumin, glucose, ketones
and infection. Blood measurements taken at the first antenatal appointment act
as baselines to measure changes (i.e. anaemia) during pregnancy. Blood pressure
is monitored. Blood samples are taken to assess blood type and Rhesus disease
(see Chapter 13).
In pregnancies with a higher risk
of anomalies or with indications of anomalies, further tests may be offered.
The quadruple blood test can be performed in the second trimester between 15
and 18 weeks; this tests for maternal levels of alpha‐fetoprotein (AFP), human
chorionic gonadotropin (hCG), unconjugated estriol (uE3) and inhibin A. In
combination with factors taken from the mother’s history the results of this
test may be used to create an indication of the probability of the foetus
having Down syndrome. A combined test that describes the association of the
nuchal translucency value of the foetus with maternal blood levels of free
beta‐hCG and pregnancy‐associated plasma protein‐A (PAPP‐A) at the time of the
first‐trimester scan performs the same function. The quadruple test is used if
the mother would like to determine the risk of anomalous development but is
unable to use the combined test. Invasive testing, such as chorionic villus
sampling, amniocentesis and cordocentesis, may be offered in cases that
indicate the need for further diagnostic testing. These methods have risks
associated with miscarriage but provide early diagnosis that can aid early
termination decisions.
The second‐trimester ultrasound
scan, sometimes called the ‘anomaly scan’ is offered at 18–21 weeks. At this
stage the foetus is at a size and development stage that can be examined in
enough detail to identify most anomalous development. The sonographer will
proceed through a checklist of anatomical structures to examine including the
skull, central nervous system, heart, thorax, gastrointestinal tract,
urogenital systems and the skeleton. The location of the placenta will be
determined and the volume of amniotic fluid recorded.
Antenatal appointments for low
risk pregnancies will continue at 25 weeks, then every 3 weeks assessing growth
by fundus height (Figure 48.4) until 34 weeks and then every 2 weeks until 40
weeks by which point birth would be expected. In the later stages of pregnancy
the assessment of the lie and presentation of the foetus (Figure 48.5) becomes
more important. After 41 weeks’ gestation team will discuss the induction of
labour or an lective caesarean section with the mother.
