What is a pregnancy? A question for programs of in vitro fertilization

Pregnancy outcome in studies of normal reproduction and in programs of in vitro fertilization (IVF) is usually classified as 'chemical β-human chorionic gonadotropin (β-HCG) abortion', 'trimester abortion', and 'term delivery'. The distinction between a chemical β-HCG abortion and a first-trimester abortion is not clearly stated in the literature, although such terms are commonly used. It is proposed that in programs of IVF pregnancy outcome be classified as 'menstrual abortion', 'preclinical abortion', 'clinical abortion', or 'viable pregnancy'. Pregnancy outcome of 190 consecutive cycles induced by human menopausal gonadotropin/human chorionic gonadotropin in the program of IVF at Norfolk is compared with contemporary studies of pregnancy outcome in normal reproduction. The in vitro data indicate that the Norfolk program has recorded no menstrual abortions, a 33% preclinical and clinical abortion rate, and a viable pregnancy rate that approaches but does not equal the term delivery rate of normal reproduction. However, these results have been achieved by the transfer of multiple concepti, whereas normal reproduction depends on the fertilization of a single oocyte.