The lining of the uterus, or endometrium, undergoes specific changes after approximately five days of exposure to progesterone, creating an environment conducive to embryo implantation. During IVF procedures, embryos are typically transferred around this time, known as Day 5, when the endometrium is most receptive. This characteristic state of the endometrium is termed the "window of implantation."

For most women, this window closes by Day 6 of progesterone exposure. WOI testing involves an endometrial biopsy to assess the receptivity of the endometrium for embryo implantation. It analyses the expression of various genes that facilitate implantation and the suppression of genes that may hinder it.

A recent study, conducted as a randomized controlled trial involving patients undergoing their first IVF cycle, investigated the efficacy of WOI testing. Half of the participants underwent WOI testing, while the other half did not. Surprisingly, the results showed no significant benefit from undergoing IVF with WOI testing. This finding suggests that for most women, whose WOI typically occurs on Day 5 after progesterone exposure, WOI testing did not alter the timing of embryo transfer.

However, there may still be potential benefits for a specific subset of women: those who have undergone multiple embryo transfers using high-quality or genetically normal embryos without success. Although data supporting this notion are pending publication, WOI testing is currently considered an optional add-on treatment for IVF.

For couples or women who have experienced three or more unsuccessful embryo transfers using good quality and/or genetically normal embryos, an alternative endometrial receptivity test called ERPeak is offered at Care Fertility. While further research is needed to fully validate its effectiveness, ERPeak holds promise as a tool to optimise the timing of embryo transfer in select cases.