SCREENING TESTS IN PRENATAL CARE
Screening tests in prenatal care
On completion of this activity you should:
 be able to state that the dating scan should take place between 11 –13
weeks and 6 days
 be able to explain the reason why an exact date for the start of the
pregnancy must be known
 be able to state that an anomaly scan takes place between 18+0 and 20+6
weeks
 be able to describe the meaning of a screening test
 be able to describe the meaning of a diagnostic test
 be able to describe, in simple terms, the process of amniocentesis and CVS
 be able to explain that the results of a screening test will categorise a
pregnancy in terms of risk
 have an understanding that screening tests are offered to the mother, they
do not have to be taken
 be able to draw a graph from a given data set with the correct labels, units
and scales.
The difference between a screening test and a diagnostic test
In a diagnostic test a positive result means that the patient has the disease or
condition. Screening tests are designed to estimate the risk of the patie nt
having the disease or condition.
Diagnostic tests often require expensive equipment and highly trained
technicians to carry out elaborate testing and it can take time to get the
results.
Screening tests can be quick and easy to carry out , and the results are often
available far more quickly than those for diagnostic tests. The aim of a
screening test is to assess the probability of the foetus suffering from an
abnormality. However, screening tests have more chance of being wrong:
there are ‘false-positives’ (the test states the patient has the condition when
he/she doesn’t) and ‘false-negatives’ (the patient has the condition but the
test states he/she doesn’t).
Screening tests in prenatal care.
The average length of a normal pregnancy is 38 weeks following conception.
As ovulation usually occurs 14 days after the last menstrual period (LMP), by
convention this is generally referred to as 40 weeks as this is the length of
time that would normally have passed since the LMP .
ANTENATAL SCREENING (H, HUMAN BIOLOGY)
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SCREENING TESTS IN PRENATAL CARE
All women should be offered a minimum of two ultrasound scans during
pregnancy. At the first antenatal appointment women should be offered an
early ultrasound scan for gestational age assessment. This scan should ideally
take place between weeks 11 and 13+6 (13+6 weeks = 13 weeks and 6 days).
Provided the pregnancy is not too advanced by the time women are seen at a
hospital, all women should be offered at least two opportunities to screen for
abnormalities. It is important that it is appreciated that these tests are
‘offered’; it is an individual’s choice to decide whether or not to have them
performed.
Down’s syndrome
Screening for Down’s syndrome can be offered between 11 and 14 weeks ’
gestation by a combination of ultrasound measurement of the f oetal neck
(nuchal translucency) along with blood tests (serum hCG and PAPP-A) from
the woman. When combined these have the potential to identify about 90% of
all pregnancies affected by Down’s syndrome if every women who is
identified as being higher risk then chooses to have a diagnostic test.
If this opportunity is missed, another screening test for Down ’s syndrome can
be offered between 15 and 20 weeks using blood tests only (hCG , inhibin,
oestradiol and AFP). This test is not quite as accurate but has the potential to
identify about 70% of cases of Down’s syndrome if all women who have a
higher chance result opt to have it.
The results of all these tests – whether biochemical or nucal translucency –
are reported not in actual values but as ‘multiples of median’ (MoM). The
median is an average result for women of similar gestation , so a ‘normal’
result is 1 MoM. If the levels are higher than normal then the result will be
higher than 1 MoM.
The results are amalgamated into a single estimation of risk or chance , for
example there is a 1:150 chance of Down’s syndrome. Women who have a
chance of greater (or worse than) 1:250 are considered to have had a positive
screening test result and are offered a diagnostic test.
Ultrasound
Ultrasound screening for foetal anomalies should be routinely offered,
normally between 18+0 weeks and 20+6 weeks, although it can be undertaken
at other opportunities if there are particular reasons . For example, it may be
performed later if the woman is obese and therefore the baby is more difficult
to see or earlier if the woman has had a baby with a malformation previously .
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ANTENATAL SCREENING (H, HUMAN BIOLOGY)
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SCREENING TESTS IN PRENATAL CARE
At first contact with a healthcare professional, women should be given
information about the purpose and implications of the anomaly scan to enable
them to make an informed choice as to whether or not to have the scan. The
purpose of the scan is to identify foetal anomalies and allow:
 parents to prepare (for any treatment/disability/palliative care/termination
of pregnancy)
 managed birth in a specialist centre
 intrauterine therapy
 reproductive choice (termination of pregnancy) .
Women should be informed of the limitations of routine ultrasound screening
and that detection rates vary by the type of f oetal anomaly, the woman’s body
mass index and the position of the unborn baby at the time of the scan.
Human chorionic gonadotrophin (hCG)
hCG is a two-chain protein molecule produced by the placenta. In a normal
pregnancy the levels decrease between the 15th and 22nd weeks. In the case
of Down’s syndrome, the values are expected to be higher than the median.
Alpha fetoprotein (AFP)
AFP is a single-chain glycoprotein produced by the foetal liver. In normal
pregnancies the levels increase significantly between the 15th and 22nd
weeks of pregnancy. In the case of Down’s syndrome the levels are expected
to be lower than the median.
Unconjugated oestrol (uE3)
uE3 is a steroid hormone produced by the placenta. Chemicals produced by the
foetal adrenal gland are converted to uE3 by the placenta. In normal
pregnancies the levels increase between the 15th and 22nd weeks. (They do
not increase as much as AFP levels.) In the case of Down’s syndrome the
levels are expected to be lower than the median.
WHO criteria for screening
The condition sought should be an important health problem fo r the
individual and community.
There should be an accepted treatment or useful intervention for
patients with the disease.
The natural history of the disease should be adequately understood.
There should be a latent or early symptomatic stage.
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SCREENING TESTS IN PRENATAL CARE
There should be a suitable and acceptable screening test or
examination.
Facilities for diagnosis and treatment should be available.
There should be an agreed policy on whom to treat as patients.
Treatment started at an early stage should be of more benefit tha n
treatment started later.
The cost should be economically balanced in relation to possible
expenditure on medical care as a whole.
Case finding should be a continuing process and not a once and for all
project.
Wilson JMG, Jungner G, Principles and practice of screening for
disease. Geneva: WHO; 1968
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ANTENATAL SCREENING (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011
SCREENING TESTS IN PRENATAL CARE
Questions
The following table shows the results of a study to discover the relationship
between maternal weight and levels of AFP, hCG and uE3.
Average levels of marker (MoM)
Weight of mother
(kg)
<49
50–59
60–69
70–79
80–89
90–99
>100
AFP
hCH
uE3
1.39
1.12
1.01
0.96
0.81
0.76
0.66
1.18
1.00
1.12
0.91
0.94
0.88
0.83
1.09
1.07
1.00
1.01
0.93
0.90
0.83
1.
Present these results as a graph.
2.
Explain why it is important for laboratories to take into account the
mother’s weight when calculating risk.
3.
The graphs below show the frequency distribution of hCG and AFP
levels in unaffected and Down’s syndrome pregnancies. Where the
graphs cross is the MoM value at which the risk is the same as th e
background risk in the population.
© Nicholas Wald, Wolfson Institute of Preventative Medicine, Barts and the London
School of Medicine, Queen Marcy University of London
(a)
(b)
(c)
What is the most common hCG level in a Down’s syndrome pregnancy?
What is the most common AFP level in a Down’s syndrome pregnancy?
Why does a hCG level of 1 MoM not exclude the risk of a Down ’s
syndrome pregnancy?
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SCREENING TESTS IN PRENATAL CARE
Answers
2.
As the weight of the mother increases the MoM levels of each marker
decrease. If weight is not taken into account there is a danger of an
error in risk calculation.
3.
(a)
3 MoM
(b)
0.7 ± 0.1 MoM
(c)
From the frequency graph, there are a few Down ’s syndrome
pregnancies that had a hCG level of 1 MoM or less.
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ANTENATAL SCREENING (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011