The Menstrual Cycle and Ovulation
The menstrual cycle is the monthly series of changes in the ovaries and uterus that prepares the body for pregnancy, and ovulation is the release of a mature egg from an ovary around the middle of the cycle. In brief: a cycle averages 28 days, though cycles of about 21 to 35 days are common. Day 1 is the first day of menstruation (bleeding), ovulation typically occurs around day 14 in a 28-day cycle, and the hormones FSH, LH, estrogen and progesterone control the stages.
The cycle is controlled by feedback between the brain and the ovaries. This guide explains the phases of the cycle, the role of each hormone, what happens at ovulation, how pregnancy changes the cycle, how to read a hormone graph in a worked example and which mistakes are common.
What is the menstrual cycle?
The menstrual cycle begins at puberty (the first period is called menarche) and continues until menopause, usually between ages 45 and 55. It happens in two linked parts: the ovarian cycle, in which an egg matures and is released, and the uterine cycle, in which the lining of the uterus (the endometrium) thickens and is shed if there is no pregnancy.
The cycle length is counted from the first day of one period to the first day of the next. Cycles vary between women and from month to month, especially in the first years after menarche. Cycles are interrupted by pregnancy and breastfeeding.
What are the phases of the cycle?
Menstruation (days 1 to about 5)
The thickened lining of the uterus breaks down and is lost through the vagina, together with blood. The bleeding usually lasts three to seven days, and the total fluid lost is typically 30 to 80 milliliters. At the same time, a new set of follicles begins to develop in the ovary.
Follicular phase (days 1 to about 13)
The pituitary gland releases follicle-stimulating hormone (FSH), which causes several follicles to grow. Usually one becomes dominant. The growing follicle releases estrogen, which causes the uterine lining to thicken again and repair itself.
Ovulation (around day 14)
A peak in luteinizing hormone (LH) triggers the release of the egg from the dominant follicle.
Luteal phase (days 15 to 28)
The empty follicle becomes a structure called the corpus luteum, which produces progesterone and some estrogen. Progesterone maintains the thickened, blood-rich lining of the uterus. If no embryo implants, the corpus luteum breaks down after about 10 to 14 days, hormone levels fall and the lining is shed, which begins a new cycle.
Which hormones control the cycle?
- Gonadotropin-releasing hormone (GnRH): from the hypothalamus; stimulates the pituitary to release FSH and LH.
- FSH (follicle-stimulating hormone): from the pituitary; stimulates follicle growth and estrogen production.
- LH (luteinizing hormone): from the pituitary; a surge triggers ovulation and the formation of the corpus luteum.
- Estrogen: from the follicles; repairs and thickens the uterine lining. Moderate levels inhibit FSH (negative feedback). A high, sustained level just before ovulation stimulates the LH surge (positive feedback).
- Progesterone: from the corpus luteum; maintains the lining and inhibits FSH and LH, which prevents new follicles from developing during the luteal phase.
This system includes both negative and positive feedback, which makes it a good example of hormonal control. Hormonal contraceptives use the same pathways: the hormones inhibit FSH and LH and prevent ovulation.
What is ovulation?
Ovulation is the release of a secondary oocyte (the egg, still surrounded by cells) from the surface of the ovary. It is caught by the finger-like ends of the oviduct. The egg can be fertilized for only about 12 to 24 hours after release. Sperm may survive in the female tract for up to about five days, so the “fertile window” is roughly the five days before ovulation and the day of ovulation itself.
Some women notice signs of ovulation: a clear, stretchy cervical mucus, mild pain on one side of the abdomen, or a small increase in basal body temperature of about 0.2 to 0.5 °C after ovulation, caused by progesterone.
Phases of a 28-day cycle
| Days | Phase | Ovary | Uterine lining | Main hormone |
|---|---|---|---|---|
| 1 to 5 | Menstruation | New follicles begin to grow | Shed | Low estrogen and progesterone |
| 6 to 13 | Follicular | Follicle matures | Rebuilds | Estrogen rising |
| 14 | Ovulation | Egg released | Thick | LH surge |
| 15 to 28 | Luteal | Corpus luteum forms | Maintained | Progesterone high |
What happens if the egg is fertilized?
If fertilization occurs, the embryo travels to the uterus and implants in the lining about 6 to 10 days after ovulation. It begins to produce a hormone called human chorionic gonadotropin (hCG), which keeps the corpus luteum active, so progesterone remains high, and the lining is not shed. Menstruation stops during pregnancy. Pregnancy tests detect hCG in urine.
Worked example: reading a hormone and temperature record
A woman records her basal body temperature each morning for a 28-day cycle. Days 1 to 13 give readings of about 36.4 °C, then there is a small dip on day 14 and from day 15 to day 28 the readings are about 36.8 °C. Interpret the pattern.
- Temperature rise: 36.8 − 36.4 = 0.4 °C. This is within the usual range of 0.2 to 0.5 °C and happens after ovulation, when progesterone is released by the corpus luteum.
- Timing: the dip and rise suggest that ovulation occurred around day 14.
- Length of luteal phase: days 15 to 28 are 14 days, within the typical 10 to 16 days.
- Hormones: LH peaked before day 14, estrogen peaked on about days 12 and 13, and progesterone is high from day 15 to about day 25.
- Prediction: if the temperature stays high after day 28, this may indicate pregnancy, because progesterone remains high.
This method is not reliable as a method of contraception, because it detects ovulation only after it has happened, and the temperature is affected by illness and poor sleep. The reasoning shows how a physical sign reflects hormone levels.
How was the cycle discovered?
The Austrian gynecologist Hermann Knaus and the Japanese physician Kyusaku Ogino independently showed in the 1920s that ovulation occurs about two weeks before the next period. Estrogen and progesterone were isolated in the late 1920s and 1930s. The first contraceptive pill, based on the control of this cycle, was approved in the United States in 1960, and Gregory Pincus, John Rock and Min Chueh Chang are credited with its development.
Which mistakes are common?
Mistake 1: Assuming that ovulation always happens on day 14
It happens about 14 days before the next period, so it varies with the cycle length.
Mistake 2: Thinking that menstruation is the start of fertile time
Menstruation is the shedding of the lining. The fertile window comes around ovulation.
Mistake 3: Mixing up the roles of FSH and LH
FSH stimulates the growth of follicles, and LH causes ovulation.
Mistake 4: Saying that the egg lives for several days
An egg can be fertilized for only about a day after release. It is sperm that can survive for several days.