Antenatal diagnosis of neural tube defects in Canada: extension of a collaborative study.

Experience with the diagnosis of neural tube defects from alpha1-fetoprotein (AFP) concentrations in amniotic fluid is reported from a prospective study of five laboratories testing for 13 Canadian genetic centres. The results of the study indicate that antenatal diagnosis of open neural tube defects is being carried out effectively in Canada (in 99.2% of cases the AFP measurements were interpreted correctly). Amniocentesis should be recommended to women at high risk for having a child with a neural tube defect (i.e., those who have a child, a parent or a sibling with a neural tube defect). The rate of neural tube defects in 182 high-risk pregnancies was 2.2% for an open defect and 1.1% for a closed defect, whereas the rate in 673 pregnancies in which amniocentesis was being performed for other reasons was 0.3%. This suggests that the AFP concentration should be measured in any sample of amniotic fluid collected for other reasons (usually fetal karyotyping). There were three instances of false-negative results, for a rate of 0.4%. Two closed neural tube defects were not detected; this limitation of the test has also been found by others. One of the six fetuses with an open neural tube defect, who died in utero, had a large myelocele in the neck that was not recognized. There were also four instances of false-positive results, for a rate of 0.5%. The findings suggest that AFP values that are more than 2 but less than 7 standard deviations (SDs) above the mean may indicate a neural tube defect, and that values 7 or more SDs above the mean very likely indicate such a defect, although other reasons for such high values (e.g., fetal erythrocytes in the amniotic fluid, intrauterine death and mistaken gestational age) must be ruled out by other methods.