OBJECTIVETo compare the obstetric and perinatal outcomes between fresh embryo transfer (ET) and frozen-thawed ET (the "freeze-all" strategy) and evaluate the benefits of the "freeze-all" embryo strategy for young patients.

METHODSWe reviewed a total of 2091 ET cycles performed between January, 2011 and December, 2015 in women aged 20-35 years, including 1295 fresh ET cycles and 796 frozen-thawed ET cycles. The demographic characteristics, ovarian stimulation syndrome, clinical pregnancy rates, live birth rate and the obstetric outcomes (gestational age, preterm delivery rate and mean birth weight) were compared between the two groups.

RESULTSThe mean age of the patients receiving frozen-thawed ET cycles had a significantly younger age than those having fresh ET cycles (29.5 vs 30.2 years, P<0.05); the patients undergoing frozen-thawed ET cycles also had significantly higher estradiol level on the day of trigger (12 973 pmol/L vs 8673 pmol/L) and a greater oocyte number retrieved (12.7 vs 8.7). The incidence of severe ovarian hyperstimulation syndrome was significantly lower in patients with frozen ET than those with fresh ET (P<0.05). No significant differences were found in the pregnancy rate (59.5% vs 56.0%; P>0.05), live birth rate (50.3% vs 47.0%; P>0.05), mean birth weight or gestational age between the two groups.

CONCLUSIONSThe freeze-all policy produces similar pregnancy and obstetric outcomes with those of fresh ET. Our results support the hypothesis that the freeze-all strategy help to prevent OHSS with a good pregnancy rate.