| During the average 28 day menstrual cycle, the uterine lining, or endometrium is built up under the influence of estrogen and progesterone, which are released in response to the pituitary hormones, luteinizing hormone and follicular hormone. The proliferative phase lasts from 7 to 21 days. During the secretory phase, the uterine lining is well vascularized and suited for implantation and growth of a blastocyst, or fertilized egg. | | | | |