Effect of blastocyst morphology on the incidence of monozygotic twinning pregnancy after single blastocyst transfer

Aims: To explore whether blastocyst morphology [blastocyst stage, inner cell mass (ICM), and trophectoderm (TE) grading] impacts the occurrence of monozygotic twinning (MZT) after single blastocyst transfer (SBT).Materials and methods: A single-center retrospective cohort study was conducted including all clinical pregnancies after single blastocyst transfer between January 2015 and September 2021 (n = 9229). Blastocyst morphology was assessed using Gardner grading system. MZT was defined as more than one gestational sac (GS), or two or more fetal heartbeats in a single GS via ultrasound at 5-6 gestational weeks.Results: The overall MZT rate was 2.46% (227 of 9229 cases), of which was the highest in blastocysts of grade A TE and lowest in those with grade C TE (grade A: B:C = 3.40%:2.67%:1.58%, p = .002). Higher risk of MZT pregnancy was associated with higher trophectoderm grading [A vs. C: aOR, 1.883, 95% CI 1.069-3.315, p = .028; B vs C: aOR, 1.559, 95% CI 1.066-2.279, p = .022], but not extended culture in vitro (day 5 vs. day 6), vitrification (fresh vs. frozen-thawed ET), assisted hatching (AH), blastocyst stage (stage 1-6) or ICM grading (A vs. B).Conclusions: We conclude that TE grade is an independent risk factor of MZT after single blastocyst transfer. Blastocysts with high-grade trophectoderm are more liable to obtain monozygotic multiple gestation.