ABSTRACT
The issue of multiple pregnancy is a predictable consequence of improvements in embryo laboratory quality (and hence embryo quality) and embryo transfer technique. If the implantation rate (IR) per embryo increases from 10% to 20%, the calculated expectation of twins with two embryos transferred would increase from 1% to 4%. Likewise, if 100% of the transfer medium is retained in the desired location the chance of multiple implantation would be higher than if 50% is expelled. Clearly, higher-quality programs will have more multiples, resulting in a greater need to restrict embryo number. Conversely, programs with less refined laboratory conditions and/or transfer techniques have fewer multiples and will have a marked reduction in their success rates if they restrict embryo number for transfer. Following this logic, adherence of all in vitro fertilization (IVF) programs to restrictive criteria for the number of embryos transferred can be expected to widen the variation in success rates among programs. It will allow some programs to more clearly identify suboptimal techniques since a low pregnancy rate demands more attention than a low implantation rate.
