Evaluation of clinical parameters as predictors of monozygotic twins after single frozen embryo transfer

OBJECTIVE: To determine if recent evolutions in laboratory protocols, including the increased use of natural cycles and the use of a hyaluronan-containing transfer medium, affected the rate of monozygotic twin (MZT) pregnancies after single frozen embryo transfer (FET). DESIGN: Retrospective cohort study. SETTING: Urban university-based fertility center. PATIENT(S): Patients who underwent single FET between January 2016 and December 2018 resulting in an intrauterine pregnancy. INTERVENTION(S): Transition to a transfer protocol with a hyaluronan-containing transfer medium in July 2017. MAIN OUTCOME MEASURE(S): Number of MZT pregnancies. RESULT(S): There were 1,619 cycles that met the inclusion criteria and 31 (1.9%) resulted in MZT pregnancies. A hyaluronan-containing transfer medium was used in 875 (54.1%) cycles. Programmed cycles were used for 1,385 (85.5%) FETs and 234 (14.5%) cycles were natural. The mean age at FET, oocyte age, endometrial echo thickness, inner cell mass grade, trophectoderm grade, expansion, and day of blastocyst vitrification were similar between the groups. The use of a hyaluronan-containing transfer medium resulted in fewer MZTs. After controlling potential confounders with a multivariate regression, the use of the hyaluronan-containing medium still resulted in fewer MZTs. Monozygotic twins were colinear with preimplantation genetic testing (PGT), so PGT was excluded as a variable in our regression. A regression of PGT only cycles showed that the use of the hyaluronan-containing medium was still associated with a reduction in MZT pregnancies. CONCLUSION(S): The use of a hyaluronan-containing transfer medium was associated with a lower rate of MZTs. Other clinical parameters, including cycle type, were not associated with changes in the number of MZTs. The use of PGT needs to be further investigated as a risk factor for MZTs.