In cycles of in vitro fertilization, it is routine to stimulate the female’s ovaries so that many eggs will be obtained. Many eggs are needed in order to provide a good chance of success because most human eggs do not become viable embryos.
In many cases, we obtain more than one good embryo to consider for transfer back to the uterus of the female parent or a gestational carrier. The availability of multiple good embryos allows us to grade them and then choose the best among them for transfer. It also brings on a common dilemma – how many embryos to transfer. Some patients prefer the transfer of multiple embryos in order to increase their chance of at least one embryo resulting in a live birth.
A critical statistic in IVF is the implantation rate. The implantation rate is the proportion of transferred embryos that actually implant in the uterine lining and continue to develop so that a fetal heartbeat is detected. It is therefore the ratio of fetal hearts to transferred embryos.