Frozen embryo transfer (FET) has increased in prevalence amongall ART cycles. As use increases it is important to identify factors that improve outcomes. Several known predictors include age,number, and quality of frozen embryos. Another vital aspect is the ‘window of implantation’1, the period when the endometrium is most receptive.2 Progesterone is essential for support of the endometrium and implantation of an embryo in the normal menstrual cycle. This state is induced through an interplay of hormones including estrogen and progesterone.3 When performing FETs an attempt is made to replicate the natural hormonal environment.

GnRH agonist is administered for pituitary down regulation which in turn inhibits the secretion of LH and disturbs endogenous production of progesterone. As the endogenous production of progesterone requires a functioning corpus luteum that is not present in programed cycles., Therefore progesterone supplementation is recommended in these women as luteal phase support to make endometrium secretory and maintain pregnancy4.Hormone replacement therapy (HRT) comprising of both estradiol and progesterone was given to the patients.

According to previously established studies, progesterone luteal support increases the clinical outcome of FETs. Patients who received progesterone luteal support had higher clinical and biochemical pregnancies with increased chances of live birth and lower miscarriage rate.

Even though the adequate level of progesterone on the day of embryo transfer has an undeniable effect on clinical outcomes of FET, there are growing pieces of evidence regarding the nonlinearassociation between serum progesterone levels and such outcomes. There is an optimal window for progesterone values in luteal phases that is in controversy with the traditional belief that higher values of progesterone will lead to better clinical outcomes in FET. If the progesterone level is above an optimum level, this may decrease the pregnancy rate. It might be due to premature luteinization and its subsequent impairment in receptivity of the endometrium.5-8 This study is undertaken to determine the association of serum estradiol and progesterone levels one day prior to FET with the clinical pregnancy rate.