Incidence of dichorionic diamniotic twins after single blastocyst transfer

Abstract

OBJECTIVE: Dichorionic diamniotic twins (DDT) are common after multiple embryo transfer. However, the incidence of DDT after single embryo transfer has not been studied. The objective of this study is to investigate the incidence of DDT after single blastocyst transfer. DESIGN: Retrospective clinical study. MATERIALS AND METHODS: A total of 12,258 single blastocyst transfers were carried out at our center from May 2002 to March 2006. Procedures included 11,408 cycles of frozen blastocyst transfer and 850 cycles of fresh blastocyst transfer. Ovulation induction was performed by minimal stimulation protocol with clomiphene citrate and low dose hMG injection. Assisted hatching was performed in selected cases. Monozygotic twins (MZT) and DDT were diagnosed by transvaginal ultrasound at early gestation with numbers of gestational sacs and numbers of heart beats. It was then confirmed after delivery with number of newborns and the distinction of sex. The incidence of MZT and DDT was reported based on the final live birth outcome. The Chi-Square test was used for statistical analysis, p<0.05 is considered statistical significant. RESULTS: Overall, there were 4,704 clinical pregnancies and 3,519 live-births from 12,258 single blastocyst transfers. The implantation rate and live-birth rate were 38.4% and 28.7% respectively. There were 35 incidences of MZT in 3,519 live-birth pregnancies (0.99%). There were 11 incidences of DDT (0.31%). All DDT were resulted from frozen blastocyst transfer. We found that assisted hatching did not affect the incidence of DDT. CONCLUSIONS: It has been reported that MZT occurs in 0.42% of all birth in natural conception. In previous reports, incidence of MZT with IVF varies 2% to 8%. The high incidence was observed after blastocyst transfer. Our data showed that the incidence of MZT is 0.99%, and DDT is 0.31% from single blastocyst transfer cycles. In previous reports, MZT were studied with multiple embryo transfer cycles and inevitably, DDT were considered as fraternal twins. Our data demonstrated for the first time that the accurate incidence of DDT after single blastocyst transfer is 0.31%. It appears that frozen and thaw process may contribute the rare event. The specific mechanism is remained to be studied.