Individual implantation rate: Proposal for a new index for evaluation of assisted reproduction results

Objective: To assess whether implantation in assisted reproductive technology (ART) cycles is a random event. Design: Retrospective analysis of results. Setting: Division for Reproductive Endocrinology and ART, Department of Obstetrics and Gynecology, Haemek Medical Center, Afula, Israel. Patient(s): A cohort of all cycles reaching ET from July 1, 1995, through June 30, 1996, and a cohort of all pregnancies recorded from January 1, 1995, through October 31, 1996. Intervention(s): None. Main Outcome Measure(s): Rate of multifetal pregnancy in relation to overall pregnancy rate. The number of gestational sacs observed by sonography, out of transferred embryos in conception cycles, was defined as the individual implantation rate. Result(s): Of 367 ETs, 75 (20.4%) yielded pregnancies, of which 31 (41%) were multifetal. Considering the mean number of embryos transferred (3.67), if implantation would have been random, multifetal gestation rate should have been only 14.8%, significantly less than the observed rate. In 110 pregnancies recorded between January 1995 and October 1996, individual implantation rate was 49.4% ± 27.1% in intracytoplasmic sperm injection cycles compared with 40.5% ± 20.4% in IVF cycles. Conclusion(s): Embryo implantation is not a random event. The index of individual implantation rate may help shed light on mechanisms underlying implantation.