Double trouble? Clinic-specific risk factors for monozygotic twinning.

OBJECTIVE: To investigate clinic-specific risk factors for monozygotic twinning (MZT) using a large, electronic database. DESIGN: Retrospective case-control study. SETTING: Infertility clinics. PATIENT(S): Using an electronic medical record system, viable clinical pregnancy (confirmation of a gestational sac(s) and presence of at least one fetal pole with a heartbeat on first trimester ultrasound), data were obtained from homologous in vitro fertilization (IVF) cycles after single ET from June 1, 2004, to December 31, 2016. Monozygotic twinning was defined as a pregnancy with two fetal heartbeats on ultrasound with sex concordance at birth. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): Risk factors for MZT including cycle type, method of insemination, and method of cryopreservation. RESULT(S): Of the 28,265 IVF cycles that met inclusion criteria over the study period, 8,749 (31.0%) resulted in a viable intrauterine clinical pregnancy. There were 102 (2.7%) MZT in the fresh cycle cohort and 133 (2.7%) in the frozen cycle cohort. Neither cryopreservation nor the method of cryopreservation was a significant risk factor for MZT. However, the use of sequential media was an independent risk factor for MZT in fresh, but not frozen, ETs (odds ratio = 1.72, 95% confidence interval, 1.10-2.68). Significant differences were seen in the incidence of MZT between clinics, and this difference persisted after controlling for known risk factors (clinic 0, reference; clinic 2, odds ratio = 2.22; 95% confidence interval, 1.48-3.32; clinic 3, odds ratio = 1.93; 95% confidence interval, 1.30-2.87). CONCLUSION(S): Differences in MZT rates exist between individual IVF clinics, suggesting that variations in practice patterns may contribute to this event. The present study noted the use of sequential media was an independent risk factor for fresh but not frozen cycles.