Frozen embryo transfer (FET) cycles have become more common in recent years due to a push towards elective single embryo transfer (SET). While it is known that progesterone supplementation during the luteal phase improves clinical pregnancy rates, there is a paucity of prospective data on the impact of serum progesterone levels on pregnancy outcomes in FET cycles. This multicentre prospective cohort study aims to investigate the association between serum progesterone levels on the day of FET and pregnancy outcomes, and to determine a serum progesterone cut-off value above which clinical pregnancy and live birth are more likely to occur. Women undergoing ART-FET cycles at CARE Fertility clinics in the UK will be recruited and their serum progesterone measured on the day of frozen embryo transfer. Follow-up data will be stored in electronic patient records and analysed to determine whether a low serum progesterone level on the day of FET adversely affects ART outcomes.

Inclusion Criteria:

• All women undergoing ART with FET.

Exclusion Criteria:

• None. We aim to study the "every day" patient undergoing FET, and for this we intend to keep the population of the study as unselected as possible.

All women undergoing ART with FET.

No IPD will be shared with other researchers. Any IPD will be confined to the purposes of the study.