Objective: To evaluate the incidence of monozygotic twinning (MZT) in pregnancies conceived after blastocyst transfer compared to cleavage-stage transfer.
Design: Retrospective study.
Setting: University IVF program.
Patient(s): All IVF patients with viable pregnancies conceived during a 4-year period.
Intervention(s): Blastocyst transfer or day 3 ET.
Main outcome measure(s): Incidence of MZT assessed by transvaginal ultrasound.
Result(s): There were 11 incidences of MZT in 197 viable pregnancies (5.6%) with blastocyst transfer compared to 7 of 357 viable pregnancies (2%) with day 3 ET. In 10 of 18 pregnancies, MZT was observed in the setting of a higher order multiple gestation (6 of 11 for blastocyst transfer and 4 of 7 for day 3 ET). In the day 3 ET group, assisted hatching or intracytoplasmic sperm injection (ICSI) did not increase MZT (4 of 213, 1.9%) compared to cycles without zona breaching (3 of 144, 2.1%). Similarly, in the blastocyst-transfer group, ICSI did not increase the incidence of MZT (4 of 74, 5.5% for ICSI and 7 of 123, 5.7% for non-ICSI IVF).
Conclusion(s): Compared to day 3 ET, blastocyst transfer appears to significantly increase the incidence of gestations with MZT. This information should be taken into account when counseling patients about the pros and cons of extended culture.