Maternal serum alphafetoprotein in pregnancy and the prevention of birth defect

Screening by measurement of the maternal serum alphafeto protein is the first "genetic" tool that could be applied to the entire pregnant population, so the recent report from a working group on maternal serum alphafetoprotein screening2 prompts us to attempt to put into perspective, firstly, an assess ment of the benefits and risks of maternal serum alphafeto protein screening programmes in averting the birth of affected babies; secondly, the value that such programmes may coincidentally have in identifying high-risk but salvageable fetuses; and, thirdly, the possibility of true prevention of neural tube defect by supplementing the mother's diet. Alphafetoprotein and the fetus?Alphafetoprotein is a normal fetal protein that passes into the amniotic fluid, rising to a peak at the end of the first trimester and then declining rapidly. The alphafetoprotein derived from the fetus can be detected in the maternal serum in concentrations about 100-fold less than in the amniotic fluid. The serum concentrations continue to rise until the third trimester and then fall to normal non-pregnant concentrations at delivery.3 Brock and Sutcliffe4 first reported high concentrations of amniotic alphafetoprotein in association with open neural tube defect and this has been repeatedly confirmed.5-7 The concentration of the protein in the mother's serum also rises with a fetal neural tube defect; but there are several other causes for such a rise, including other fetal malformations, multiple pregnancy, threatened abortion, intrauterine death, and (frequently) incorrect assessment of gestational age. When a raised value is found during pregnancy further investigations are carried out, including a repeat estimation and an ultrasound scan to verify the gestational age. In the absence of any other cause for a raised concentration, amniocentesis is performed. A raised amniotic alphafetoprotein concentration is found in about 10% of these cases and usually means that a fetal defect is present, most often an open neural tube defect. In the remaining 90% of