Double trouble? Impact of frozen embryo transfer on the monozygotic twinning rate: a retrospective cohort study from 8459 cycles

Purpose: To compare monozygotic twinning (MZT) rates in patients undergoing fresh embryo transfer (ET) and frozen embryo transfer. Methods: All clinical pregnancies after single ET carried out in our IVF center between 2014 and 2018 (n = 8459) were retrospectively analyzed for the incidence of MZT. MZT rate was compared in women who underwent fresh ET (n = 3876) and frozen ET (n = 4583). Results: There was a total of 120 MZT identified in the fresh ET group (3.10%) and 103 MZT in the frozen ET group (2.25%), which was significant (p = 0.015). In the univariate analysis, the risk of MZT was decreased with frozen embryo transfer (OR 0.72; 95% CI, 0.55–0.94, p = 0.016) and increased with mild stimulation protocol in the fresh cycle (OR 1.90; 95% CI, 1.04–3.45, p = 0.036). Multivariable logistic regression revealed that frozen embryo transfer was associated with a significant decrease risk of MZT (adjusted OR 0.66; 95% CI, 0.46–0.90, p = 0.011). Conclusions: Frozen ET is associated with a lower risk of MZT.