Towards single births after assisted reproduction treatment.

The cause of increases in recorded multiple births is undoubtedly assisted human conception. Many arise when three or more embryos are replaced after IVF or ISCI. Others are due to multi-ovulation during artificial insemination or intrauterine inseminations, since complete control over numbers of ovulated eggs following ovarian stimulation is not fully possible. The frequency of multiple IVF births is best controlled by reducing numbers of transferred embryos to one or two. Methods to induce ovarian stimulation in amenorrhoeic and then in cyclic women 30 years ago have been greatly refined and extended in current practice. Curiously, a great many of them produce oocytes which fail to implant after fertilization, whether natural or induced cycles are being used. This massive failure of most embryos to implant poses numerous questions on the evolution of this situation in humans.