After zygote formation, embryonic development begins. This includes cleavage, morula formation, blastula formation, implantation, gastrulation and organogenesis. Implantation happens on 8^th day after fertilisation and this is considered to be the beginning of pregnancy. This continues till parturition begins. This is maintained because the shedding of the endometrium is prevented as progesterone and estrogen levels are maintained and this is due to secretion of human chorionic gonadotrophin. Thus if progesterone receptors are disrupted then uterus will not be able to support pregnancy as it will not respond to progesterone.