Menstruation and ovulation result from of a very complex and delicately balanced chemical cycle that you go through every month or so.
We'll start with your hypothalamus, which is a gland in your brain. It sends chemical messengers all over your body to control many body functions, including eating, sleeping and menstruating. The hypothalamus is sensitive to stress and other things going on in your life, so just as you might be unable to sleep if you are excited or upset, your menstrual cycle may also be influenced by your moods.
The hypothalamus keeps track of the hormone levels in your blood. At day one of your menstrual cycle, the first day of your period, the levels of estrogen, an important hormone for women, become very low. The hypothalamus responds by telling the pituitary gland, which is also in the brain, to send two kinds of hormones into the bloodstream. One hormone is called Lutenizing Hormone (LH) and the other is called Follicle Stimulating Hormone (FSH). These two hormones are made especially to encourage your ovaries to produce an egg.
When the LH and FSH arrive at the ovaries, they stimulate the growth of 10 to 20 follicles. A follicle is a little bundle of cells that contains an undeveloped egg. Follicle means "bag" or "sack" in Latin. You have around 400,000 follicles in your ovaries when you are born, more than enough for a lifetime.
As the follicles grow, they start to produce the hormone estrogen. The estrogen causes the lining of the uterus - the endometrium - to begin to grow again. Remember, this started on the first day of the cycle, when the bleeding began. The uterus lining is completely shed now, and needs to start growing again.
Around day 14, one of the follicles is bigger than all the others. It holds the egg that will be released this month The estrogen levels are very high at this point. The pituitary responds by decreasing the FSH it has been releasing, but increasing the LH. This jump in the level of LH is what causes the giant follicle in the ovary, now called the Graafian follicle, to burst open and explode right through the ovary wall, releasing the egg. This is ovulation.
The ends of the Fallopian tubes are covered with little wavy fingers called fimbria. The fimbria pull the newly released egg up into the Fallopian tube, where it will spend 12 to 36 hours moving toward the uterus. It is during this time that fertilization could occur if it meets a sperm. Only one ovary is stimulated per cycle, and one egg is released. Once in a while two eggs are released, and this results in fraternal twins, twins who are not identical because they came from two eggs and two sperm.
Back in the ovary, the exploded follicle still has work to do. It is now called the corpus luteum, Latin for "yellow body" because it turns yellow after it releases the egg. It now becomes a temporary kind of gland. It continues to make estrogen, but it really pumps out the hormone progesterone ("the pregnancy hormone") The progesterone makes the new uterine lining rich with nutrients needed to support an embryo.
Progesterone also signals the pituitary to stop sending LH, which is what keeps the corpus luteum going. So the corpus luteum only lasts about twelve days total, unless the egg is fertilized. As the corpus lutem begins to die, it stops making progesterone and estrogen. Without these hormones the lining of the uterus begins to weaken, tiny blood vessels in it shut down, and it begins to shed off. This shedding is menstruation. The uterus sheds two thirds of its lining at each menstruation, leaving only the bottom layer to grow again.
One the first day of menstruation, estrogen levels are very low, and
the hypothalamus signals the pituitary gland to send off LH and FSH, and
it all begins again.