What is Menstruation?
Menstruation is the female reproductive cycle marked by uterine bleeding in response to hormonal change. Over the cycle the ovum matures and a new uterine bed is prepared for implantation of a fertilized ovum.
Characteristics
What normal flow looks like:
- Average age at onset: 12.4 years.
- Usual range of onset: 9-17 years.
- Average interval between cycles: 28 days; 23-35 days is also normal.
- Average duration of flow: 2-7 days, with 1-9 days not unusual.
- Average volume: 30-80 mL.
- Normal color: dark red, a mix of blood, mucus, and endometrial cells.
- Normal odor: likened to marigolds.
Physiology
- The cycle is initiated by luteinizing hormone-releasing hormone, released mainly by the hypothalamus.
- At puberty the hypothalamus becomes less sensitive to estrogen feedback, which triggers release of gonadotropin-releasing hormone (GnRH).
- GnRH travels from the hypothalamus to the pituitary, signaling release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
- FSH matures the ovum. LH drives ovulation and thickens the uterine lining.
- FSH activates one primordial follicle to grow and mature. At full maturity it is the Graafian follicle.
- The pituitary releases LH, and prostaglandins rupture the Graafian follicle. Ovulation occurs and the ovum is swept into the fallopian tube.
- LH acts on the ovarian follicle cells to produce a yellow fluid, lutein, which fills the empty follicle (now the corpus luteum).
- Without conception, the unfertilized ovum atrophies after 4 to 5 days and the corpus luteum persists for 8 to 10 days, then is replaced by the corpus albicans, a white fibrous tissue.
The Phases of the Menstrual Cycle
Proliferative Phase
- After 4 to 5 days, the uterine lining is one cell layer deep and very thin.
- The ovary produces estrogen and the endometrium proliferates to roughly eightfold its usual thickness.
- From day 5 to 14, the endometrium keeps thickening.
Secretory Phase
- LH drives progesterone formation in the corpus luteum, giving the endometrium a twisted appearance.
- Capillaries increase and the lining becomes rich and spongy.
Ischemic Phase
- Without fertilization, the corpus luteum regresses by its tenth day.
- Progesterone and estrogen fall, the uterine lining sloughs off, and capillaries rupture.
- The discharge contains blood, mucus, endometrial tissue, and the unfertilized ovum. Menses is the only external marker of the cycle and marks day one of the next cycle.
Menstrual Disorders
Dysmenorrhea
Painful menstruation. Prostaglandins released during the ischemic phase cause the contractions and pain.
Menorrhagia
Abnormally heavy flow, greater than 80 mL. Without subsequent progesterone secretion and ovulation, estrogen production continues and the endometrium overproliferates.
Metrorrhagia
Bleeding between periods, usually from decreased progesterone and endometrial sloughing.
Mittelschmerz
Pain when follicular fluid drops into the abdominal cavity and releases prostaglandins. It is a clear ovulation marker because the pain is felt on one side of the abdomen, near the ovary.
Endometriosis
Extrauterine endometrial cells grow abnormally on the uterine ligaments or ovaries. Excessive endometrial production plus backflow of blood and tissue into the fallopian tube during the cycle drives it.
Absence of menstrual flow from a non-definitive cause can occur in any female who is pregnant or has other related conditions.
Frequently Asked Questions
What does a normal menstrual cycle look like?
The average interval is about 28 days, with 23 to 35 days also normal. Flow lasts 2 to 7 days, and the average volume is 30 to 80 mL. Average age at onset is about 12.4 years, within a usual range of 9 to 17 years.
What hormones drive the cycle?
The hypothalamus releases gonadotropin-releasing hormone, which signals the pituitary to release follicle-stimulating hormone and luteinizing hormone. FSH matures the ovum, and LH drives ovulation and thickens the uterine lining.
What are the three phases of the cycle?
The proliferative phase, when estrogen rebuilds and thickens the endometrium; the secretory phase, when progesterone from the corpus luteum makes the lining rich and spongy; and the ischemic phase, when hormones fall, the lining sloughs, and menses begins.
What is the difference between menorrhagia and metrorrhagia?
Menorrhagia is abnormally heavy flow, greater than 80 mL, usually from continued estrogen without progesterone. Metrorrhagia is bleeding between periods, usually from decreased progesterone and endometrial sloughing.
What is dysmenorrhea, and what causes it?
Dysmenorrhea is painful menstruation. Prostaglandins released during the ischemic phase cause the uterine contractions and the cramping pain.
Why does ACOG treat the menstrual cycle as a vital sign?
Because abnormal menstrual patterns in adolescence can be an early signal of underlying health concerns. Tracking cycle timing and flow alongside the other vital signs helps catch problems sooner.