Abstract: Objective：To analyze the pregnancy outcome of patients with severe ovarian hyperstimulation syndrome (OHSS) and the related risk factors. Methods：The pregnancy outcomes of 240 pregnant women with severe OHSS from January 2010 to September 2016 were analyzed. 510 pregnant women matched by age and body mass index (BMI) after in vitro fertilization (IVF) without OHSS were selected as the control group. Results：① Compared with the control group, the severe OHSS group had higher number of antral follicle count (AFC) and higher level of serum anti Mullerian hormone (AMH) and higher proportion of long-protocol used in ovulation induction. The level of estrogen on the hCG day in the severe OHSS group was significantly higher than that in the control group, and the numbers of dominant follicles and retrieved oocytes were higher than those in the control group (P＜0.05). ② The average number of embryos transferred and the twin rate in the severe OHSS group were higher than those in the control group, while the gestational weeks and birth weight of newborns were lower than those in the control group (all P＜0.05). There was no significant differences in the incidences of preterm delivery, gestational diabetes mellitus (GDM), pregnancy induced hypertension (PIH) and premature rupture of membranes (PROM) between the two groups (all P＞0.05). ③ Pregnancy outcomes of single pregnancy and twin pregnancy were analyzed respectively. There were no significant differences in the incidences of GDM and PROM, cesarean section rate, birth weight of newborn, gestational week of delivery and premature delivery rate between the two groups (all P＞0.05). There was no significant difference in the incidence of PIH in single fetus pregnancy between the two groups (P＞0.05). However, the incidence of PIH in twin pregnancy in the severe OHSS group was significantly lower than that in the control group (P＜0.05). Conclusions：The decreased gestational age and birth weight after severe OHSS pregnancy may be associated with the increased twin pregnancy ratio. Controlling the number of embryos transferred in the fresh cycles can effectively reduce the occurrence of OHSS, and can improve the pregnancy outcome of OHSS patients.

Key words: Ovarian hyperstimulation syndrome, Pregn, Pregnancy outcome, Twins, Embryo transfer