Progesterone, produced by the corpus luteum, causes the endometrium of the uterus to thicken and become vascular during the luteal phase of the ovarian cycle. Increased production of progesterone causes the endometrium to double or triple in size during this phase. Estrogen, also produced by the ovaries, causes the endometrium to thicken and become vascular during the follicular phase of the ovarian cycle. Menstruation begins on day one of the ovarian cycle when female sex hormone levels are low. During pregnancy, the placenta produces both estrogen and progesterone. These hormones inhibit the release of gonadotropic hormones by the anterior pituitary, so no new follicles mature.