Prenatal diagnosis and genetic screening : community and service implications / a report of the Royal College of Physicians.

  • Royal College of Physicians of London.
Date:
1989
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Prenatal diagnosis and genetic screening : community and service implications / a report of the Royal College of Physicians. Source: Wellcome Collection.

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    Screening for congenital malformations Infectious causes Maternal serum alphafetoprotein estimation Ultrasound scanning 2.9 The Rubella virus is the main recognised infectious cause of congenital mal formations. Maternal immunity should always be ensured before pregnancy, but in practice this is not always the case. Positive evidence of maternal infection in early pregnancy often leads directly to therapeutic abortion, although only about 20% of exposed fetuses are severely affected. Definitive prenatal diagnosis of affected fetuses is possible by measuring the appropriate antibodies in fetal blood, but only at about 22 weeks’ gestation when the fetal immune system is sufficiently mature [5]. In future it may be possible to diagnose or exclude fetal rubella infection in the first trimester of pregnancy by testing for virus DNA in chorionic villus samples [6]. 2.10 Alphafetoprotein (AFP) is a fetal plasma protein. When a malformation such as a neural tube defect allows it to leak into the amniotic fluid, some also passes into the maternal circulation. In many areas maternal serum AFP estimation is now offered routinely to all pregnant women between 16 and 18 weeks of gestation in order to identify a risk group that includes all fetuses with anencephaly and most with spina bifida. 2.11 The service is usually organised from the antenatal clinic, but AFP screening- can be carried out in collaboration with general practitioners. Information and counselling for the women who are tested and evaluation of outcome are essential. Results of maternal serum AFP assay are expressed quantitatively as deviations from a median value, and vary with the assay method used, the population studied and gestational age. Laboratories participate in a quality-control programme and regularly monitor their results. Ultrasound is an important adjunct to maternal serum AFP screening. 2.12 A raised maternal serum AFP should lead to expert (level 3) ultrasound examination for a fetal malformation, with or without amniocentesis for confirm atory biochemical tests, or else directly to amniocentesis for assay of the amniotic fluid AFP. It is important to note that in about 50% of pregnancies with a raised maternal serum AFP, no cause can be found, either pre- or post-natally [7]. Maternal serum AFP estimation is likely to remain an important screening test because the expert routine ultrasound scanning for fetal abnormalities, which some think could replace it, is not generally available and because when integrated with maternal age a low maternal serum AFP is a useful screen for fetal chromosomal abnormality (see para 3.3). 2.13 Ultrasound scanning is now a basic part of obstetric practice. There is a continual increase in the range and capabilities of the best equipment and a decrease in the size and cost of basic machines. The use of ultrasound in screening and prenatal diagnosis will be discussed together here, because essentially the same methods are used for both, though practised at different levels of clinical expertise. 2.14 There is good evidence that with an appropriately organised obstetric ultra sound service, most major structural malformations could be detected in the second trimester of pregnancy (at about 19 weeks’ gestation), allowing parents the option of termination of pregnancy. These findings need confirmation in larger studies. 2.15 There is no evidence for a harmful physical effect of diagnostic obstetric ultrasound [8]. Its main limitations are the dependence on the skill and experience of the operator and the quality of the equipment, and its main risk is misinterpretation of the image leading to failure to detect abnormalities (false negatives) or to abortion of a healthy fetus (false positives).