The prenatal diagnosis of neural tube defects.

Abstract

This review concentrates on the current status of alpha fetoprotein (AFP) measurement in the early prenatal diagnosis of spina bifida and anencephaly. High incidence rates are found in Britain and Ireland intermediate ones in Europe North America and Israel and low ones in Africa Asia and South America. Risk of recurrence for each pregnancy subsequent to the birth of a neural tube defective child is 1/20. Amniotic fluid AFP in normal pregnancies parallel the fetal serum levels with a concentration gradient of about 150:1 so the highest concentration of about 40 mcg/ml is found at 13-14 weeks gestation. Immunodiffusion techniques are adequate for qualitating levels above 1 mcg/ml. As a diagnositc test applied before 20 weeks of pregnancy amniotic fluid AFP is very specific. Elevated levels have also been associated with twin pregnancy severe RH immunization and in third trimester fetal distress. Other possible abnormalities for which AFP may be diagnostic include Turners syndrome and various other chromosomal and congenital abnormalities. In addition increased bilirubin beta-trace protein and macroglobuins may indicate early neural tube defects. Ultrasonography has also been proved useful for prenatal diagnosis of neural tube defects. For measuring maternal serum AFP immunodiffusion techniques are not sensitive enough and specific radioimmunoassays must be employed. 80-90% of anencephalic and 40-50% of open spina bifida defects may be determined by maternal serum screening.