Introduction
insufficiency, OHSS
OVARIAN HYPERSTIMULATION SYNDROME
In today’s IVF practice, frozen embryo transfer (FET) cycles have started to override fresh transfers, owing to the improvements in vitrification techniques along with the added benefits of elimination of ovarian hyperstimulation syndrome (OHSS) and improved success rates in hyper-responders when freeze-all strategy is used. Increased utilisation of preimplantation genetic screening of embryos further augments the switch to frozen transfer cycles (The corpus luteum (CL) is involved in a natural cycle FET. Nevertheless, progesterone production by CL in some cycles may be insufficient to provide adequate support for implantation and continuation of pregnancy. A very recent meta-analysis of randomised controlled trials for luteal phase support (LPS) in natural cycle FET (NC-FET) has shown that progesterone supplementation for LPS was associated with increased LBR and CPR in NC-FET cycles, giving additional evidence for the insufficiency of corpus luteum in some of the natural cycles (The purpose of our trial is to evaluate whether initiating SC progesterone supplementation on the day of embryo transfer when serum progesterone levels are below 10 ng/mL in tNC-FET will result in pregnancy rates comparable to those of patients with sufficient serum progesterone. In brief, to our knowledge, this is the first study to evaluate the feasibility of a rescue progesterone supplementation strategy in tNC-FET cases.