Frozen versus fresh embryo transfer is associated with a lower risk of monozygotic twinning in blastocysts

What is the relationship between frozen embryo transfer (FET) and risk of monozygotic twinning (MZT)?

This large multicentre retrospective cohort study analysed data from 36,771 assisted reproductive cycles conducted at five academic hospitals across China between January 2013 and February 2020. MZT was defined as the presence of more than one fetal heartbeat in a single gestational sac or the number of gestational sacs exceeding the number of transferred embryos. The multivariable Poisson regression model was employed to estimate the relative risk (RR) and 95% confidence interval (95% CI) for MZT associated with different types of embryo transfer.

The overall MZT rate was 1.40% (514/36,771), with rates of 1.66% (218/13,111) in the fresh embryo transfer group and 1.25% (296/23,660) in the FET group. Multivariable Poisson regression revealed that FET was associated with a 44% reduced risk of MZT (RR 0.56, 95% CI 0.42-0.76) compared with fresh embryo transfer. After stratifying by embryo stage, FET was associated with a 41% reduced risk of MZT (RR 0.59, 95% CI 0.42-0.83) compared with fresh embryo transfer only at the blastocyst stage. In addition, FET was associated with a decreased risk of MZT (RR 0.54, 95% CI 0.37-0.81) compared with fresh embryo transfer in blastocysts with a low grading of both inner cell mass and trophectoderm.

FET carried a lower risk of MZT at the blastocyst stage for the group with a low grading of both inner cell mass and trophectoderm compared with fresh embryo transfer. FET may decrease the risk of MZT by influencing blastocyst morphology.