Early identification of fetal abnormalities is possible as a result of improved ultrasound resolution, chorionic villus sampling, and early genetic amniocentesis. A potential advantage of early genetic amniocentesis over chorionic villus sampling is its ability to detect neural tube defects. We obtained 476 amniotic fluid samples between ten and 15 weeks' gestation and analyzed them for karyotype and alpha-fetoprotein (AFP); 142 were also tested for acetylcholinesterase. Amniotic fluid AFP levels rose to a peak at 12-13 weeks' gestation and then gradually declined, closely approximating the pattern in fetal blood. The rate of inconclusive acetylcholinesterase results (a faint but true band) was four times higher than that later in pregnancy (10.6 versus 2.46%, respectively). However, the rate of associated fetal congenital anomalies was lower than that later in pregnancy. Chromosomal abnormalities were detected in the study group, and the association with low amniotic fluid AFP in early genetic amniocentesis levels was identical to that further along in pregnancy. These data help establish normal values for AFP in early pregnancy. With AFP and cautious interpretation of acetylcholinesterase, early genetic amniocentesis can be used for neural tube defect detection.