Does assisted hatching pose a risk for monozygotic twinning in pregnancies conceived through in vitro fertilization?

Objective: To examine the association between assisted hatching and monozygotic (MZ) twinning. Design: Case-control. Setting: Population-based sample of IVF-ET cycles initiated in U.S. clinics, 1996. Patient(s): The IVF-ET (n = 35,503) cycles and 11,247 resultant pregnancies. Intervention(s): Use of an assisted hatching procedure on embryos transferred. Main Outcome Measure(s): Cases were pregnancies for which number of fetal hearts observed on ultrasound exceeded number of embryos transferred. These pregnancies were considered to contain at least one MZ set of twins. Cases were compared with two control groups: other multiple-gestation pregnancies (greater than or equal to 2 fetal hearts but number of fetal hearts less than or equal to number of embryos transferred); and singleton pregnancies (1 fetal heart). Result(s): Women with a case pregnancy were more likely to have received embryos treated with assisted hatching procedures than were women in either control group. After adjustment for patient age, number of embryos transferred, prior cycles, infertility diagnosis, intracytoplasmic sperm injection, and whether embryos from the current cycle were cryopreserved for later use, odds ratios and 95% CIs for use of assisted hatching were 3.2 (1.3-8.0), compared with other multiple-gestation pregnancies, and 3.8 (1.8-9.8), compared with singleton pregnancies. Conclusion(s): Assisted hatching may pose a risk for MZ twinning. (C) 2000 by American Society for Reproductive Medicine.