Girls are born with about two million oocytes, or eggs. By the time a girl reaches puberty, she only has about 400,000 eggs left. A young girl's first period is called menarche and occurs on average around age twelve. Of course, there is great variation, as some girls start menstruating earlier while others won't begin until as late as age sixteen or seventeen.

The cessation of menses is a process called menopause, which typically happens around age fifty. As you reach menopause, the end of the fertile phase of your life, you will have very few eggs left. During the course of your fertile periods, you will have only produced about 400 mature eggs. The years leading up to menopause are called perimenopause and can start up to ten years before the last period.

The average length of a menstrual cycle — which is measured from the start of one period to the start of the next — is twenty-eight days, but it can be as short as twenty-one days and as long as thirty-five days. When you start to think about attempting to conceive a child, it can be helpful to track your cycles. Record the start date, how long the period is, and anything that you are concerned about. There are four stages of the menstrual cycle: the follicular, ovulatory, and luteal phases, and menstruation.

The Follicular Phase

The follicular phase is the beginning part of the cycle, when a follicle and the egg inside it begins to mature. The process begins when a special hormone, called gonadotropin-releasing hormone, is released from the hypothalamus. That hormone does exactly what its name implies — it causes gonadotropins to be released from the pituitary gland, another small gland in the brain. These gonadotropins, called follicle stimulating hormone (FSH) and lutenizing hormone (LH), stimulate follicular growth in the ovaries.

As the follicle develops, it secretes small amounts of a form of estrogen, called beta-estradiol, until the egg reaches maturity. At that point, the pituitary starts pumping out large amounts of LH, a surge that precedes ovulation by approximately thirty-six hours. Ovulation predictor kits work by detecting these elevated levels of LH.

Ovulation

Ovulation, or the release of a mature egg from the ovary, occurs approximately fourteen days into a cycle. Every cycle is different though, and you may ovulate on day sixteen one month and day thirteen the next. For that reason, it can be helpful to start testing for your LH surge a few days earlier then you expect ovulation to take place if you are using an ovulation predictor kit. You don't want to start too late and miss your surge.

The empty egg follicle is now known as the “corpus luteum” and it produces a hormone called progesterone.

Contrary to popular belief, your ovaries do not necessarily take turns ovulating. In fact, some women will always ovulate from one ovary. Which ovary is chosen each month depends on where the dominant follicle resides. The ovary with the dominant follicle is the lucky one to release an egg that month.

You may be able to tell when you are ovulating without charting your menstrual cycle. Some women report feeling a small pain or cramp on one side or the other as the egg is released. This is called “mittelschmerz.”

The Luteal Phase

The luteal phase is the second half of the cycle and is characterized by the presence of the corpus luteum (empty egg follicle) that produces large amounts of progesterone, the primary hormone of pregnancy in case of a possible pregnancy. If pregnancy does happen, eventually the placenta will take over as the progesterone producer and the corpus luteum will degenerate. The levels of progesterone in the body will wane if pregnancy does not occur, and it is this falling progesterone level that triggers the next menstrual period.

While all of these changes are occurring with the egg, the uterine lining is undergoing several changes as well. The lining is growing progressively thicker and becoming more developed in anticipation of the implantation of a fertilized egg.

Menstruation

If an embryo does not implant, the body sheds the uterine lining, expelling the blood, tissue, and egg as menses, or a woman's period. Hormone levels return to their baseline level and prepare for the start of the next cycle. Gonadotropin-releasing hormone is produced by the hypothalamus and the cycle starts again.

TABLE 2-1: HORMONAL LEVELS DURING THE PHASES OF THE MENSTRUAL CYCLE