Basic Endocrinology

Menstrual Cycle

The menstrual cycle involves a complex interplay of hormones and physiological responses over an approximate monthly period. The hormones involved include estrogen, luteinizing hormone (LH), follicular stimulating hormone (FSH), and progesterone.

FSH, a hormone secreted by the pituitary gland, stimulates the growth and maturation of the egg prior to its release from the ovary. This hormone promotes the development of the follicle, a shell of tissue that surrounds the egg. When the estrogen produced by the ovaries reaches its plateau, the hormone induces LH production by the pituitary gland which, in turn, triggers the release of the egg through the follicle. The egg then travels down the fallopian tube towards the uterus, or womb. After ovulation, the follicle breaks down into a yellow body, or "corpus luteum", and begins to produce progesterone. The progesterone halts the release of all other eggs until the following cycle and maintains the thick lining of the uterus in preparation for pregnancy. If the egg does become fertilized, it occurs in the fallopian tube within hours of ovulation. It is thought that the fertilized egg is then drawn toward the uterus via hairlike projections, called cilia, lining the fallopian tubes. Within approximately one week, the fertilized egg implants at the endometrium, the innermost layer of the uterus. On the other hand, if the egg is not fertilized as it moves down the fallopian tube, then progesterone production diminishes causing the endometrium to shed and pass through the vagina, a process called menstruation.

Most women begin menstruating between the ages of 8 and 18 years old, and stop menstruating between the ages of 40 and 60. The menstrual cycle lasts an average of 28 days and ovulation occurs on day 14 in most women. During their menstruation period, women can bleed anywhere from 3 to 7 days.