ObjectiveTo determine whether there is a gender discrepancy in severe twin-twin transfusion syndrome.MethodsAll cases of twin-twin transfusion syndrome evaluated between 1989 and 1996 were reviewed retrospectively. The following sonographic criteria were used: a single placenta, a thin membrane, the same gender, a combination of polyhydramnios-oligohydramnios, a stuck twin, and an estimated weight discordance exceeding 20%. At least five of six sonographic criteria were required for inclusion in the study. Only severe cases, which were defined as early onset (before 30 weeks' gestation), a combination of polyhydramnios and oligohydramnios, a stuck twin, fetal hydrops, fetal death, or the requirement of medical or invasive treatment, were included. Chorionicity was confirmed by placental examination when available.ResultsThirty-seven twin pregnancies met the above criteria, of which 33 (89%) twin pairs were female. The median gestational age at presentation was 19 weeks (range, 15–29; standard deviation, 5.6). A single placenta, thin membrane, same gender, and polyhydramnios-oligohydramnios were present in every case. A stuck twin was noted in 34 of 37 cases (92%), and a growth discordance exceeding 20% was present in 26 of 36 (72%). Placental pathology, which was available in 31 (84%) cases, confirmed a monochorionic placentation in 29. Twenty-five (68%) cases had reduction amniocentesis, two were treated with indomethacin, one underwent a cord ligation, and in four cases, fetal death occurred before treatment was instituted.ConclusionThere is a significant female preponderance in pregnancies complicated by severe twin-twin transfusion syndrome. The reasons for this are unclear, but they may be related to either placental or fetal gender-specific differences affecting a subset of monochorionic twin pregnancies.