Objective: To determine whether there is a gender discrepancy in sever
e twin-twin transfusion syndrome. Methods: All cases of twin-twin tran
sfusion syndrome evaluated between 1989 and 1996 were reviewed retrosp
ectively. The following sonographic criteria were used: a single place
nta, a thin membrane, the same gender, a combination of polyhydramnios
-oligohydramnios, a stuck twin, and an estimated weight discordance ex
ceeding 20%. At least five of six sonographic criteria were required f
or inclusion in the study. Only severe cases, which were defined as ea
rly onset (before 30 weeks' gestation), a combination of polyhydramnio
s and oligohydramnios, a stuck twin, fetal hydrops, fetal death, or th
e requirement of medical or invasive treatment, were included. Chorion
icity was confirmed by placental examination when available. Results:
Thirty-seven twin pregnancies met the above criteria, of which 33 (89%
) twin pairs were female. The median gestational age at presentation w
as 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 reduc
tion amniocentesis, two were treated with indomethacin, one underwent
a cord ligation, and in four cases, fetal death occurred before treatm
ent was instituted. Conclusion: There is a significant female preponde
rance in pregnancies complicated by severe twin-twin transfusion syndr
ome. The reasons for this are unclear, but they may be related to eith
er placental or fetal gender-specific differences affecting a subset o
f monochorionic twin pregnancies. (C) 1997 by The American College of
Obstetricians and Gynecologists.