The first phase is the follicular phase, or the proliferative phase. The follicular phase begins on Day 1 of the menstrual cycle. Estrogen and progesterone levels are at their lowest during menstruation. During this phase the uterine lining, or endometrial lining, both sheds through menstruation and begins a period of regrowth and thickening in preparation for an embryo should conception occur. The follicular phase lasts about 10 to 14 days, or until ovulation occurs.
The second phase of the menstrual cycle called the luteal phase begins when ovulation occurs. During ovulation, the ovaries release a single egg from only one of the two ovaries during each menstrual cycle. Ovulation is a process that begins when the level of luteinizing hormone or LH surges, and ends 16 to 32 hours later with the release of an egg from the ovary.
The luteal phase continues until Day 1 of your next menstrual period. Your levels of estrogen and progesterone rise during the luteal phase. These hormones work together to cause changes to the endometrial lining that prepare it for an embryo should conception occur. When conception, or pregnancy, does not occur, levels of estrogen and progesterone decrease causing the endometrial lining to shed through menstruation.
Source: Menstrual Cycle Biology of the Female Reproductive System; Merck Manual; accessed 07/20/07.
Menstrual Cycle Biology of the Female Reproductive System; Merck Manual; accessed 07/20/07.