The data retrieved for each patients is not unique for the study but includes the basic variables monitored in every IVF cycle : Estradiol serum levels (E2), Progesterone serum levels (P), number of follicles as counted by trans-vaginal ultrasound, endometrial thickness as measured by trans-vaginal ultrasound, Follicle stimulating hormone (FSH) dosing, number of stimulation days, number of eggs retrieved, fertilization rate, blastulation rate, number of embryos transferred, embryo quality, positive pregnancy test, implantation rate, presence of a clinical pregnancy, presence of OHSS. Since these are mandatory variables in the treatment we do not expect any missing data. In addition to these mandatory data the study patients will be performing two additional blood tests on day 3 post ovum pickup and on the transfer day itself (day 5 post ovum pickup) including Estradiol serum levels (E2) and Progesterone serum levels (P).