The menstrual cycle involves menstruation, follicular/proliferative phase, ovulation, and luteal/secretory phase. Hormonal imbalances and physical factors can cause anovulation, leading to irregular or absent periods. Treatment can restore menstrual cycle functioning.
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 over 29 days.
The menstrual cycle is divided into four main phases. Menstruation is the first stage, when the lining of the uterus, called the endometrium, is shed. Blood and endometrial tissue are pushed out of the body through the vagina. Menstrual cramps are the result of the muscles pushing out the endometrium.
Once the uterine lining has been shed, the follicular or proliferative phase begins. During this stage, 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, increased estradiol levels cause the release of luteinizing hormone (LH), which causes the follicle to release a secondary oocyte, which matures into an egg or egg cell. The egg leaves the ovary, enters the fallopian tube and stays there for about a day. If it is fertilized by a sperm, it will implant itself in the endometrium and eventually mature into an embryo; otherwise, 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 implantation of the fertilized egg. If no egg is implanted, the corpus luteum atrophies and the progesterone level decreases, causing the endometrium to shed and the menstrual cycle and ovulation to resume.
Menstruation and ovulation are generally moderately regular and predictable, but a number of conditions can interfere with the relationship between menstruation and ovulation and, therefore, with the functioning of the cycle. Hormonal or chemical imbalances, low body weight, certain diseases, and other physical factors can lead to anovulation, in which menstruation occurs but a woman does not ovulate. Anovulation can cause irregular periods or stop periods altogether, a condition called amenorrhea. Alternatively, excessive bleeding may occur. Anovulation is usually treatable and treatment will restore the functioning of the menstrual cycle.
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