Monozygotic twinning following assisted conception: an analysis of 81 consecutive cases. Hum Reprod 2003;18:1937–43

BACKGROUND: This study attempts to identify risk factors for monozygotic (MZ) twinning following assisted conception. METHODS: Eighty-one MZ twinning pregnancies that occurred over a 7-year period in our IVF/ embryo transfer programme were evaluated. These were compared with 4224 non-MZ pregnancies from the same period. RESULTS: The overall incidence of MZ twinning was 1.88 % (81/4305) of all clinical pregnancies. A total of 63 % (51/81) of the MZ pregnancies included one or more other implantations as well. MZ twinning was unrelated to maternal age, paternal age, gonadotrophin dosage, peak estradiol and progesterone levels, number of oocytes collected, and number of embryos replaced. Patients with MZ twinning had signi®cantly more attempts at assisted reproduction than those with non-MZ multiple pregnancy. A logistic regression analysis suggested a role, albeit not emphatic, for the zona pellucida in MZ twinning. Twenty-four of the 65 MZ cases with known placentation were monoamniotic. This incidence far exceeds that seen following spontaneous conception (P < 0.0001). CONCLUSIONS: The risk of monoamniotic twinning is increased following IVF/embryo transfer. Zona pellucida disruption appears to increase the incidence of MZ twinning. However, the overall micromanipulation data together with the unexpected placentation data suggest that zona-mediated embryo splitting is not the only mechanism of twinning under arti®cial conditions. Key words: blastocyst/monoamniotic/monochorionic/monozygotic twinning/ovulation inductio