Diagnosis of neural tube defects by evaluation of amniotic fluid

Neural tube defect has occurred in approximately 2% of 1101 pregnancies at increased risk because of a positive history. The risk is lower than that generally given to families that currently present for NTD genetic counseling and prenatal diagnosis. In contrast, pregnancies undergoing amniocentesis subsequent to positive maternal serum AFP screening demonstrate an approximate 5.5% risk for open NTD and an additional 1.9% risk for other major fetal malformations. It appears, therefore, that a pregnancy subgroup with the highest risk for open NTD (three times greater than families with a history of NTD) can be identified by screening the unselected pregnant population. In 6504 pregnant women undergoing amniocentesis for reasons other than a history of NTD or maternal serum AFP levels, open NTD was detected in 1 of 296 cases. Such a yield certainly supports the concept that if amniotic fluid is being obtained for evaluation, added AFP testing is justified. In response to similar data, generated worldwide, the Consensus Development Conference held in 1979 by the National Institute of Child Health and Human Development concluded: 'The laboratory quantification of amniotic fluid AFP should be considered in pregnancies in which the following indications exist: previous child with NTD, parent with NTD, elevated serum AFP, parents identified as carriers of congenital nephrosis, and pregnancies undergoing midtrimester amniocentesis for other indications.' In evaluating such a variety of at-risk pregnancies, it must be recognized that AFP quantitation in both maternal serum and amniotic fluid is a nonspecific index of fetal malformation. Elevated amniotic fluid AFP can be associated with fetal demise, abdominal wall defects such as gastroschisis and omphalocele, and rare methods of visualizing the fetus are important. While amniography has in the past been helpful in identifying NTD and other fetal conditions, the technique is difficult to interpret and adds the risk of spontaneous abortion to pregnancies already at increased risk. We have increasingly relied on the greatly enhanced resolution of both real time and static ultrasound imaging of discrete fetal structure in order to determine the cause of AFP levels.