Complex interactions among hormones control the start of menstruation during puberty Puberty in Girls Puberty is a sequence of events in which physical changes occur, resulting in adult physical characteristics and capacity to reproduce. These physical changes are regulated by changes in the... read more , the rhythms and duration of menstrual cycles Menstrual Cycle Menstruation is the shedding of the lining of the uterus (endometrium) accompanied by bleeding. It occurs in approximately monthly cycles throughout a woman's reproductive life, except during... read more during the reproductive years, and the end of menstruation at menopause Menstrual Cycle Menstruation is the shedding of the lining of the uterus (endometrium) accompanied by bleeding. It occurs in approximately monthly cycles throughout a woman's reproductive life, except during... read more (which is usually defined as beginning 12 months after a woman's last period).
The hormonal interactions that control menstruation occur in the following sequence:
The hypothalamus is the part of the brain that coordinates and controls hormonal activity.
Gonadotropin-releasing hormone (GnRH) is released from the hypothalamus in pulses.
The pituitary gland (also located in the brain) is stimulated by GnRH.
Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are released by the pituitary gland.
The ovaries are stimulated by LH and FSH.
The ovaries produce the female hormones estrogen and progesterone, which ultimately control menstruation Menstrual Cycle Menstruation is the shedding of the lining of the uterus (endometrium) accompanied by bleeding. It occurs in approximately monthly cycles throughout a woman's reproductive life, except during... read more .
Hormones produced by other glands, such as the adrenal glands and the thyroid gland, can affect the functioning of the ovaries and menstruation.
Changes During the Menstrual Cycle
 | The menstrual cycle Menstrual Cycle Menstruation is the shedding of the lining of the uterus (endometrium) accompanied by bleeding. It occurs in approximately monthly cycles throughout a woman's reproductive life, except during... read more is regulated by the complex interaction of hormones: luteinizing hormone, follicle-stimulating hormone, and the female sex hormones estrogen and progesterone. The menstrual cycle has three phases:
The menstrual cycle begins with menstrual bleeding (menstruation), which marks the first day of the follicular phase. When the follicular phase begins, levels of estrogen and progesterone are low. As a result, the top layers of the thickened lining of the uterus (endometrium) break down and are shed, and menstrual bleeding occurs. About this time, the follicle-stimulating hormone level increases slightly, stimulating the development of several follicles in the ovaries. Each follicle contains an egg. Later in this phase, as the follicle-stimulating hormone level decreases, only one follicle continues to develop. This follicle produces estrogen. As the follicular phase continues, the increasing levels of estrogen cause the lining of the uterus to thicken. The ovulatory phase begins with a surge in luteinizing hormone and follicle-stimulating hormone levels. Luteinizing hormone stimulates egg release (ovulation), which usually occurs 32 to 36 hours after the surge begins. The estrogen level peaks during the surge, and the progesterone level starts to increase. During the luteal phase, luteinizing hormone and follicle-stimulating hormone levels decrease. The ruptured follicle closes after releasing the egg and forms a corpus luteum, which produces progesterone. During most of this phase, the estrogen level is high. Progesterone and estrogen cause the lining of the uterus to thicken more and thus prepare for possible fertilization. If the egg is not fertilized, the corpus luteum degenerates and no longer produces progesterone, the estrogen level decreases, the top layers of the lining break down and are shed, and menstrual bleeding occurs (the start of a new menstrual cycle). |
During the reproductive years, vaginal bleeding may be abnormal if menstrual periods are too heavy or too light, last too long, occur too often, or are irregular. Any vaginal bleeding that occurs before puberty or after menopause is considered abnormal until proven otherwise. Most causes of abnormal vaginal bleeding are not serious.
Menstrual cycle problems include
Irregular or absent menses and abnormal vaginal bleeding have many causes. During the reproductive years, pregnancy is the most common cause. Doctors evaluate vaginal bleeding in pregnant women differently from vaginal bleeding in women who are not pregnant (see Vaginal Bleeding During Early Pregnancy Vaginal Bleeding During Early Pregnancy During the first 20 weeks of pregnancy, 20 to 30% of women have vaginal bleeding. In about half of these women, the pregnancy ends in a miscarriage. If miscarriage does not occur immediately... read more and Vaginal Bleeding During Late Pregnancy Vaginal Bleeding During Late Pregnancy During late pregnancy (after 20 weeks), 3 to 4% of women have vaginal bleeding. Such women are at risk of losing the baby or of bleeding excessively (hemorrhaging). Sometimes so much blood is... read more ).
Some disorders that are related to the reproductive organs but not only to the menstrual cycle can cause some of the same symptoms as menstrual disorders. These disorders include
