Amniotic fluid alpha-fetoprotein as a marker in prenatal diagnosis of neural tube defects

Abstract

The prenatal diagnosis of anencephaly and spina bifida (neural tube defect, NTD) through amniotic fluid analysis for alpha-fetoprotein (AFP) is gradually gaining clinical recognition. AFP concentrations were determined in 237 amniotic fluids from normal pregnancies ranging between 7 and 42 weeks of gestation. A steady decline in AFP from 26 mug/ml at 7-9 weeks to 155 ng/ml at term is observed. AFP concentration was determined in 35 amniotic fluids from 33 confirmed neural tube defective pregnancies. In 14 cases where amniotic fluid was examined prior to the 26th week of gestation. AFP was markedly elevated when compared with the normal range of the same gestational period. In 21 amniotic fluids past the 26th week, 17 cases (85-) had markedly elevated AFP levels; however, 2 cases of anencephaly, 1 of spina bifida, and 1 of hydrocephaly gave levels within the normal range. It is concluded that elevated AFP in the amniotic fluid is a reliable but nonspecific marker for open neural tube defects prior to the 26th week of pregnancy, but may become normal after the 26th week in a small percentage of patients.

PIP: A steady decline in alpha fetoprotein (AFP) levels was observed in single specimens of amniotic fluid (AF) from 237 patients, ranging from 26 mcg/ml at 7-9 weeks to 155 ng/ml at term. All pregnancies tested were normal. 35 AF specimens from 33 confirmed neural tube defective pregnancies were assayed for AFP. Very high levels of AFP were found in 13/14 fluids examined before Week 26 of gestation. A value of 23 mcg/ml was determined in 1 sample where the infant had skin-covered encephalocele. A fluid taken from the same patient at 34 weeks fell to 6.4 mcg of AFP. 21 AF samples from patients past the 26th week of pregnancy were analyzed. Of these, 1 case of spina bifida and 2 of anecephaly gave no detectable levels of AFP by electroimmunodiffusion. By radioimmunoassay, however, these samples measured 3700, 256, and 700 ng/ml. 1 case of hydroencephaly, examined at 33 weeks, had an AFP level of 1.5 mcg/ml. A sharp drop in AFP from 353.6 at 15 weeks to 10.4 mcg/ml at 29 weeks was noted in the only serially examined open neural tube defective pregnancy.