One hundred twin pregnancies in a prenatal diagnosis program.

One hundred pairs of twins were encountered in 8,500 pregnancies having genetic amniocentesis. Only 5 of 27 (18.5%) pairs were recognized before the institution of routine ultrasonic examination, while 69 of 73 (94%) twin pairs were found after ultrasound use. Amniotic fluid was obtained from both sacs in 71 of the 73 (97%) identified twin gestations in which both twins were living at the time of amniocentesis.