Trisomy 21 in one of twin fetuses.

Both the detection of twins and the successful execution of a double amniocentesis pose significant technical problems in prenatal diagnosis. A case is reported in which one of twins fetuses had trisomy 21 and the other was chromosomally normal. Following counseling, the family chose to continue the pregnancy. At term, the mother was delivered of a healthy infant and a severely macerated fetus with stigmata suggestive of Down's syndrome.