When IVF started, cryobiology did not afford the opportunity to freeze embryos. Furthermore, the pregnancy rate per embryo transferred was much lower that it is today. Thus, multiple embryos were transferred so reporting the pregnancy rate by number of live births per retrieval was an accurate reflection of the chance of having a child from one IVF cycle. The risks of this approach were that stimulation treatments were designed to produce many follicles to get many oocytes. Also because the per embryo delivery rate was low, many embryos were transferred at one time resulting in risks of having triplets, quads, quints etc.
Improvements in culture media, medications, and cryobiology have dramatically changed the landscape. Now the goal is to do single embryo transfer and freeze extra embryos to be used in a frozen embryo transfer cycle. This has resulted in the development of “milder” stimulation regimens to reduce the risk of OHSS. The milder stimulation regimens, however, result in fewer oocytes being retrieved in any given cycle. Nonetheless, many couples will have enough embryos to have both a fresh and a frozen embryo transfer from a single retrieval.