Menstruation and ovulation are both part of the human female reproductive cycle, also called the menstrual cycle. In a typical reproductive cycle, menstruation and ovulation are separated by about nine days, although this varies from person to person and from cycle to cycle. The average length of the menstrual cycle is just more than 29 days.
The menstrual cycle is divided into four main phases. Menstruation is the first phase, when the lining of the uterus, called the endometrium, is shed. Blood and endometrial tissue are forced out of the body through the vagina. Menstrual cramps are the result of the muscles expelling the endometrium.
Once the uterine lining has been shed, the follicular or proliferative phase begins. During this phase, the body releases hormones that stimulate the growth of an ovarian follicle. As the follicle grows, it in turn releases a hormone called estradiol, which stimulates the regrowth of the endometrium.
Ovulation is the third phase of the cycle. As the ovarian follicle matures, rising levels of estradiol cause the release of luteinizing hormone (LH), which causes the follicle to release a secondary oocyte, which matures into an ovum, or egg. The ovum leaves the ovary, enters the fallopian tube and remains there for about a day. If it is fertilized by a spermatozoon, it will implant itself in the endometrium and eventually mature into an embryo; if not, it will disintegrate in the fallopian tube.
In the luteal phase, also known as the secretory phase, the remaining parts of the ovarian follicle transform into a body called the corpus luteum, which releases more hormones, including progesterone. These hormones make the endometrium more receptive to the implantation of the fertilized egg. If no egg is implanted, the corpus luteum atrophies and the level of progesterone falls, causing the endometrium to be shed and the cycle of menstruation and ovulation to begin again.
Menstruation and ovulation are usually moderately regular and predictable, but a number of conditions can interfere with the relationship between menstruation and ovulation and, thus, with the functioning of the cycle. Hormonal or chemical imbalances, low body weight, some diseases, and other physical factors can lead to anovulation, in which the menstrual cycle occurs but the woman does not ovulate. Anovulation can cause irregular periods or stop menstruation altogether, a condition called amenorrhea. Alternately, excessive bleeding may occur. Anovulation is usually treatable, and treatment will restore the functioning of the menstrual cycle.