Cycle regimens for frozen-thawed embryo transfer.

Pregnancy rates following frozen-thawed embryo transfer (FET) treatment have always been found to be lower than following embryo transfer using fresh embryos. Nevertheless, FET increases the (cumulative) pregnancy rate, reduces cost, is relatively simple to undertake and can be accomplished in a shorter time period compared to repeated 'fresh' cycles. FET is performed using different cycle regimens: spontaneous ovulatory cycles, cycles in which ovulation is induced by drugs and cycles in which the endometrium is artificially prepared by oestrogen (O) and progesterone (P) hormones, with or without a gonadotrophin releasing hormone agonist (GnRHa).