46,XY,i(21q) identified by maternal serum screening.

Maternal serum screening for the detection of fetal Down syndrome has become widespread. Prenatal detection of fetal Down syndrome has important implications not only for management of the current pregnancy, but also for recurrence risk counseling for future pregnancies. We report a case of fetal Down syndrome due to an isochromosome 21q detected after maternal serum screening using alpha-fetoprotein and human chorionic gonadotropin indicated an increased risk for fetal Down syndrome in a 19-year-old pregnant woman. This confirms that maternal serum screening can detect fetal Down syndrome due to rare chromosome rearrangements and illustrates the importance of cytogenetic studies for provision of appropriate genetic counseling.