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Effect of embryo transfer number on singleton and twin implantation pregnancy outcomes after assisted reproductive technology.

OBJECTIVE
To evaluate the effect of embryo transfer number on singleton and twin pregnancy outcomes with one and two fetal heartbeats on early ultrasound, respectively.


STUDY DESIGN
The study included 23,645 singleton and 14,083 twin live births from the SART-CORS Online database of assisted reproductive technology cycles for 2004-2006. The data were limited to fresh embryo transfers among women who had additional embryos cryopreserved during the same cycle, resulting either in one fetal heartbeat on early ultrasound and a singleton live birth, or two fetal heartbeats and a twin live birth. Data were categorized by number of embryos transferred; the reference group was one for singletons and two for twins. The primary outcome measure was moderate growth restriction (birthweight for gestation z-score < -1), adjusted for potential confounders.


RESULTS
Risk for moderate growth restriction among singletons was increased by 15%, 23%, and 37%, respectively, with 2, 3, and > or = 4 embryos transferred, and among twins, by 50% and 105%, respectively, with 3 and > or = 4 embryos transferred.


CONCLUSION
The data demonstrate a significant residual adverse effect on intrauterine growth from transfer of multiple embryos. Whether this effect is due to compromised embryo quality, degenerating implantation sites, or other etiology is unclear.