Clinical outcomes of dichorionic diamniotic twin pregnancies following single versus double embryo transfer in human medically assisted reproduction.

study questionDo clinical and perinatal outcomes of dichorionic diamniotic (DCDA) twin pregnancies differ between single embryo transfer (SET) and double embryo transfer (DET) in human medically assisted reproduction (MAR)? SUMMARY ANSWER: In DCDA twin pregnancies, SET was associated with a significantly higher incidence of complete miscarriage and a lower rate of twin live births than DET. WHAT IS KNOWN ALREADY: While DET has historically been the major contributor to dizygotic DCDA twins, the global adoption of SET has markedly reduced such cases. However, monozygotic twinning (MZT) occurs more frequently after MAR, especially in blastocyst transfer cycles, and the prognosis of monozygotic DCDA twins remains poorly understood. STUDY DESIGN, SIZE, DURATION: This single-center retrospective cohort study analyzed 206 clinical multiple pregnancies achieved between January 2014 and December 2024, following 4658 fresh and 15 872 frozen-warmed embryo transfer cycles. PARTICIPANTS/MATERIALS, SETTING,

methodsOnly cycles using autologous oocytes were included. Clinical and perinatal outcomes of DCDA twin pregnancies derived from SET and DET were compared. To account for baseline differences between SET and DET groups, an exploratory multivariable logistic regression analysis was performed for clinical outcomes. Statistical analyses were performed using the Mann-Whitney U-test and Fisher's exact test, with P < 0.05 considered significant. MAIN RESULTS AND THE ROLE OF CHANCE: When comparing the clinical course of DCDA twin pregnancies, the incidence of two gestational sacs and two fetal heartbeats was significantly higher in the DET group than in the SET group (98.0% vs 47.2%, P < 0.0001; 63.5% vs 25.5%, P < 0.0001) (two fetal heartbeats: adjusted odds ratios [aOR], 0.276; 95% CI, 0.108-0.706; P < 0.007). Twin live birth occurred in 53.1% of DET-derived DCDA twins and 17.6% of SET-derived DCDA twins (P < 0.0001) (aOR, 0.324; 95% CI, 0.121-0.867; P = 0.025), whereas complete miscarriage was more frequent after SET (49.0% vs 17.7%, P < 0.0001) (aOR, 9.140; 95% CI, 3.030-27.600; P < 0.0001). Perinatal outcomes, including gestational age, birth weight, and congenital anomaly rates, did not differ significantly between groups. LIMITATIONS, REASONS FOR CAUTION: The number of monozygotic cases was limited, and zygosity could not be genetically confirmed. Some same-sex DCDA twins may have been dizygotic in origin. WIDER IMPLICATIONS OF THE

findingsThese findings highlight that DCDA twin pregnancies should not be regarded as a uniform clinical entity in MAR. Even within the same chorionicity category, early outcomes differ significantly between monozygotic twins after SET and dizygotic twins after DET. Although SET remains the optimal strategy to prevent multiple pregnancies, further studies should aim to identify embryos at higher risk of post-transfer splitting and to refine preventive criteria for MZT. STUDY FUNDING/COMPETING INTEREST(S): There is no funding for this study. TRIAL REGISTRATION NUMBER: N/A.