Antenatal screening
There are approximately 700,000 pregnancies per annum in the UK and 200 to 250 million world wide. Most result in the birth of a healthy baby, though in a few cases there may be problems affecting delivery or a baby’s development. Antenatal screening is a way of assessing whether the fetus could potentially develop, or indeed has developed, an abnormality during pregnancy. If the risk is high the mother may be offered prenatal diagnosis to find out the likelihood of developing the abnormality. Prior knowledge of problems can help parents plan how best to deal with them: by preparing for special care, or choosing to terminate the pregnancy.
Overview of screening programmes
There are a range of antenatal screening programmes. Most include tests to diagnose a variety of genetic and infectious conditions, including Down’s syndrome, spina bifida, sickle-cell anaemia, thalassaemia, HIV, hepatitis B, syphilis and rubella. In general they may be considered in three groups:
(a) Fetal screening for Down’s syndrome and spina bifida.
(b) Fetal anomaly screening by ultrasonography – usually at 18–20 weeks – to identify developmental abnormalities, including congenital heart defects and cleft lip and confirm spina bifida.
In addition, women are offered screening for HIV, hepatitis B, syphilis and rubella early in pregnancy.
Screening for Down’s syndrome
In the UK, all pregnant women are offered screening for trisomy 21 (Down’s syndrome) either in the first or second trimester. The first screening test is used to estimate a risk or probability of a fetus being affected. If the risk is higher than a pre-determined cut-off then a second diagnostic test is offered, which provides a definite result. The tests are optional and women may choose to refuse or opt out of the process at any stage. Screening tests are not foolproof. A proportion of cases are missed (false negatives) and most of the ‘screen positive’ cases turn out not to have the abnormality (false positives).
First trimester screening
Although second trimester screening has been common practice, combined first trimester screening is currently considered to be best practice as it provides a higher detection rate and lower false positive rate. It uses a combination of ultrasound measurement of fetal nuchal translucency (NT), and measurement of the maternal serum markers free beta HCG (FβHCG) and pregnancy-associated plasma protein A (PAPP-A), to derive a combined risk for Down’s syndrome. Each of these markers, including NT, varies with gestation and an accurate measurement of fetal maturity is required for accurate interpretation of results. For first trimester screening, ultrasound measurement of fetal crown rump length (CRL; Fig 77.1), carried out at the same time as the NT measurement, is used as the basis of the calculation of gestation for conversion of marker concentrations into a multiple of the median (MoM). An MoM is a measure of how far an individual test result deviates from the median. MoM is commonly used to report the results of medical screening tests, particularly where the results of the individual tests are highly variable.
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