Factors affecting the outcome of assisted conception.

In order to assess the major biological and clinical factors which influence the outcome of both IVF and GIFT, the results of a large GIFT series performed in this unit have been analysed over a time interval when two different policies have been in force as to the number of oocytes that should be transferred. The reason for using GIFT data, as opposed to IVF, is simply because it is more difficult to adversely affect the outcome of GIFT treatment, as opposed to IVF, and hence this allows more critical appraisal and interpretation of results. The GIFT series to be described was conducted between March 1986 and October 1987, when a flexible policy on the number of oocytes for transfer was in operation. Since then a fixed policy has been in force at the request of the Hospital's Ethics Committee and the Voluntary Licensing Authority (now the Interim Licensing Authority), whereby three, or exceptionally four, oocytes are transferred irrespective of individual patient's circumstances because of concern about the relative risk and sequelae of multiple pregnancy and selective reduction of pregnancy. I still believe, however, that some infertile couples have a lower chance of pregnancy with assisted, or natural conception and also a lower risk of multiple pregnancy and that it is therefore important to assess the biological and clinical factors influencing fecundity in order to formulate appropriate treatment policies.