Human zona pellucida micromanipulation and monozygotic twinning frequency after IVF

To assess the association of zona pellucida micromanipulation and subsequen t development of monozygotic twins, cases of assisted embryo hatching (AH) and intracytoplasmic sperm injection (ICSI) were identified and related to treatment type, implantation and zygosity data. Embryology records from all patients undergoing in-vitro fertilization (IVF) at this centre from Janua ry 1995 to March 1998 were reviewed, In this study, 3546 transfer cycles we re completed, with clinical pregnancy established in 1911 (54% per transfer ) patients undergoing a single IVF cycle. These pregnancies occurred in 167 4 (88%) IVF cycles, 120 (6%) donor oocyte cycles (DER), and 117 (6%) frozen embryo transfer (FET) cycles. During the study period, 23 cases of monozyg otic (MZ) twins were identified, representing an overall frequency of 1.2%. Chorionicity was determined by transvaginal ultrasound at 7 weeks when the number of embryos transferred was less than the number of fetal heartbeats , or when >1 fetal heartbeat per gestational sac was seen. Zygosity was con firmed by placental evaluation at delivery, and corroborated the antenatal diagnosis in all eases, Among IVF study patients the frequency of MZ twinni ng was not statistically different between zona manipulated and zona intact subgroups. While this investigation is the largest to date describing the relationship between MZ twins and zona procedures, studies with even greate r statistical power are needed to clarify it more precisely, particularly i n DER and FET settings. A greater overall frequency of MZ twinning for IVF patients may be a function of the higher number of embryos transferred in N F, rather than discrete zona manipulations.