The aim of the study is to compare the pregnancy rate between women treated with Gonadotropin Releasing Hormone (GnRH) agonist together with Human Chorionic Gonadotropin (HCG) and standard luteal support with progesterone following transfer of frozen embryos in in-vitro-fertilization (IVF) natural cycles.

The rate of frozen embryos transfer (FET) has increased in recent years due to a higher tendency of single embryo transfer, use of preimplantation genetic testing, and prevention of ovarian hyperstimulation. There are different methods to prepare the endometrium for FET: 1. Natural cycle (NC) - natural preparation of the body for implantation without the need for medicinal intervention. 2. Modified natural cycle (mNC) - Inducing ovulation by administration of Human Chorionic Gonadotropin (HCG) trigger. 3. Medical - preparation of endometrium with hormones (estrogen and progesterone). Following embryo transfer, luteal phase support should be considered. Administration of treatment depends on the type of cycle chosen (either natural or medicated). Treatment options include progesterone, HCG and Gonadotropin Releasing Hormone (GnRH) analog - either one of them or combined. Different protocols (dosages and duration of use) with different pregnancy outcomes were explore before with inconclusive results. We wish to investigate administration of GnRH agonist+HCG vs. progesterone. Patients included in the study will be women undergoing natural cycle FET at the IVF center in Shaare Zedek, Jerusalem, Israel. All patients will be randomly divided into two groups, each will receive a different luteal treatment support as will be detailed later. Patients demographic data and pregnancy outcomes will be collected. Based on the clinical pregnancy rate in the prospective study by Bjuresten, et al \[Fertil Steril, 2011\], where a clinical pregnancy rate of 32% was found among women treated with progesterone for luteal support, and given an alpha of 5% and a power of 80%, 144 women (72 women in each group) are required in order to demonstrate a clinical pregnancy rate of 55% in the study group. Comparison of categorical variables will be carried out using the the chi-squared test. Comparison of continuous variables will be performed using the Students t-Test or Mann-Whitney U method depending on the variable distribution (normal vs. non-normal distribution, respectively). A multivariate regression will also be conducted in order to determine which variables are significantly and independently related. Statistical significance will be defined when P values are less than 0.05