One defect that should be readily sought is neural tube defects. Maternal serum alpha-fetoprotein screening between 15 and 18 weeks, now recommended in all pregnancies, allows prenatal detection of 80-90% of neural tube defects. Elevated serum values may warrant amniocentesis for definitive confirmation of neural tube defects. Given the small but finite risks of amniocentesis, the procedure should be offered for detection of neural tube defects only after maternal serum alpha-fetoprotein levels are found to be elevated. Amniocentesis or chorionic villus sampling solely for prenatal cytogenetic diagnosis is not recommended on the basis of maternal diabetes. However, if amniocentesis is performed for assessment of amniotic fluid alpha-fetoprotein, chromosomal analysis is recommended. The converse is also true.