Objective To study the perinatal outcome of monochorionic (MC) twin pregnancies.Methods The 197 cases of MC twin pregnancies delivered in obstetric unit of Prince of Wales Hospital from Jan.2005 to Dec.2010 were enrolled in this retrospective study,which were divided into 3 groups:uncomplicated MC pregnancy (136 cases),twin to twin transfusion syndrome (TTTS,32 cases),selective intrauterine growth restriction (sIUGR,29 cases).Results (1)The gestational age at delivery in TTTS (30 weeks vs.37 weeks,P ＜0.01) or sIUGR (34 weeks vs.37 weeks,P ＜0.01) group were significantly lower than those in uncomplicated MC pregnancy.The number of live-born in TTTS (36 cases)or sIUGR (47 cases) group were significantly lower than those in uncomplicated MC pregnancy (261 cases,P ＜0.01).(2) The incidence of fetal death (IUFD) in TTTS [34％ (11/32) vs.4％ (6/136),P ＜0.01]or sIUGR group [28％ (8/29) vs.4％ (6/136),P ＜0.01] were significantly higher than those in uncomplicated MC pregnancy group.(3) The incidence of birth weight (BW) disc ordance ＞ 25％ in TTTS [5/15 vs.7.7％ (10/130),P ＜0.05] or sIUGR[33％ (7/21) vs.7.7％ (10/130),P ＜0.05] group were significantly higher than those in uncomplicated MC pregnancy group.(4) The incidence of preterm delivery before 37 weeks was 49％ (66/136) and 69％ (20/29) respectively in uncomplicated MC pregnancy and sIUGR.The incidence of preterm delivery in sIUGR group were significantly higher than that in uncomplicated MC pregnancy group (P ＜ 0.05).(5) The incidence of perinatal mortality in TTTS [12％(5/41) vs.0.4％ (1/262),P ＜0.01] or sIUGR[10％ (5/52) vs.0.4％ (1/262),P ＜0.01] group were significantly higher than those in uncomplicated MC pregnancy group.(6) The incidence of estimated fetal weight (EFW) discordance ＞25％ was 17％ (23/136) in uncomplicated MC pregnancy,52％ (15/29) in sIUGR,63％ (20/32) in TTTS.In uncomplicated MC pregnancy or sIUGR,pregnancies with EFW discordance ＞25％ was associated with lower gestational age at delivery,higher incidence of preterm delivery and admission to NICU than those with EFW discordance ≤ 25％ (P ＜ 0.05).In uncomplicated MC pregnancy,pregnancies with EFW discordance ＞ 25％ was associated with higher incidence of neonatal asphyxia (Apgar score 1 minute ≤7) than those with EFW discordance≤25％ (P ＜0.05).(7) Compared to uncomplicated MC pregnancy,the incidence of umbilical artery (UmA) flow abnormality was higher in TTTS[38％(12/32) vs.9.6％(13/136),P＜0.01]or sIUGR[41％(12/29) vs.9.6％(13/136),P＜0.01].The incidence of IUFD in pregnacies with UmA flow abnormality was 2/13,6/12 and 5/12 respectively in uncomplicated MC pregnancy,TTTS and sIUGR.Conclusions MC pregnancies complicated by either TTTS or sIUGR have poorer perinatal outcome as compared with uncomplicated MC pregnancies.MC pregnancies with EFW discordance ＞ 25％ and UmA flow abnormality are associated with even worse perinatal outcome.Close fetal monitoring is needed in MC pregnancies in order to reduce perinatal mortality.