Accumulated experience with the sonographic detection of spina bifida and its associated fetal cranial changes indicates that nearly all these fetuses may be identified by ultrasound examination alone. A review of data from multiple centers shows that a complete and detailed, normal ultrasound is an appropriate basis for a reduction of at least 95% in the maternal serum alpha-fetoprotein (MSAFP)-based risk for neural tube defects. The use of this adjusted risk in patient counseling before amniocentesis may lower the rate of the procedure with no significant loss of diagnostic sensitivity. Among patients with elevated MSAFP, the rate of abnormal cytogenetic findings in fetuses with no abnormalities detected at ultrasound appears to be near 0.61%. Furthermore, the spectrum of abnormal cytogenetic results appears to differ from that associated with increased maternal age, in that the incidence of sex chromosome abnormalities is higher and that of Down syndrome is lower.