Single embryo transfer as a model for early conception and implantation

Introduction This chapter deals with single embryo transfer (SET) as a model for early conception and implantation. The idea is to shed some light on the “black box of early pregnancy” by focusing on what happens after SET and double embryo transfer (DET). As early as 1945 the vanishing twin phenomenon was recognized for the first time when it was suggested that twins were more often conceived than born [1]. The vanishing twin was described as the presence of a fetus papyraceus and additionally the author proposed that twin material could be reabsorbed due to early death without leaving any trace. Decades later, ultrasound has confirmed beyond doubt the events described by Stoeckel [1], characterized since as the “vanishing twin” phenomenon, which could also be designated as spontaneous reduction. The routine use of ultrasonography has now proved that spontaneous reduction is a relatively frequent event. The “incipient twin”, is not the same as the vanishing twin, but the very early twin gestation which cannot be diagnosed by disappearance of a twin gestation. One simple way to present a quantitative estimate of the “incipient twin” is to measure the difference between the numbers of embryos transferred and the number of implanted embryos, i.e. gestational sacs. As assisted reproductive technology (ART) has been a worldwide contributor to the general increase in twin birth rates, the ART techniques have also increased the number of singletons born after an incipient or a vanishing twin pregnancy.