Natural Frozen Embryo Transfer (FET) vs Substituted Frozen Embryo Transfer (FET)

In frozen embryo transfer substituted cycles, patients are treated with progesterone and estrogen in an attempt to mimic the natural cycle and a transfer is planned whenever the endometrium is ready. In contrast, in frozen embryo transfer natural cycle patients are not taking any hormones, only triggered with hCG (human chorionic gonadotropin) when a dominant follicle is ready or they wait for the disappearance of the dominant follicle to plan the transfer date. Some doctors and centres shift towards using the substituted protocol in an attempt to make scheduling easier with less frequent ultrasounds. This study will aim to compare clinical and biochemical pregnancy rates in patients who underwent frozen embryo transfers in natural cycles vs substituted cycles at the centre.