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

PurposeTo compare monozygotic twinning (MZT) rates in patients undergoing fresh embryo transfer (ET) and frozen embryo transfer.MethodsAll 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).ResultsThere 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).ConclusionsFrozen ET is associated with a lower risk of MZT.