Trisomy 18 and 21 in two siblings.

While a child with trisomy 21 is a generally accepted indication for amniocentesis in succeeding pregnancies, the need for this prenatal test following a child with trisomy 18 is less certain. Although the likelihood of two successive pregnancies resulting in trisomies (18 and 21) appears to be small, it may be more common than is generally realized due to the high mortality of trisomy 18 prior to diagnostic studies.