The menstrual cycle has three phases: follicular, ovulation, and luteal. Learn about what happens during each phase
The menstrual cycle has three phases based on the timing of release of the egg (ovum):
- Follicular phase: Before the release of the egg; menstrual bleeding occurs during this phase.
- Ovulation phase: Egg is released.
- Luteal phase: After the egg is released.
Typically, the length of a woman’s menstrual cycle is anywhere between 25-36 days. Only 10%-15% of women have menstrual cycles that are exactly 28 days.
The menstrual cycle begins with menstrual bleeding (menstruation), which marks the first day of the follicular phase and is counted as day one of the menstrual cycle.
What happens during the follicular phase?
- Menstrual bleeding: Menstrual bleeding marks the beginning of the follicular phase, where the body sheds the innermost layers of the uterine wall (endometrium).
- Development of follicles: This is the main event of the follicular phase that starts after menstruation ends:
- Production of follicle-stimulating hormone (FSH) increases. This hormone stimulates the growth of 3-30 follicles in the ovary. Each follicle contains an egg.
- Later, FSH levels decrease as only one follicle, which is the dominant follicle, grows and produces estrogen and the other follicles break down. Estrogen helps prepare the uterus for any pregnancy that ensues.
- Duration: The average duration of the follicular phase is about 13-14 days. It varies considerably from woman to woman and tends to become shorter when a woman approaches menopause.
What happens during the ovulation phase?
- Surge of luteinizing hormone and release of the egg: The surge of luteinizing hormone (LH) marks the beginning of the ovulatory phase. LH stimulates the dominant follicle to finally rupture and release the egg.
- Fertilization: If the egg is released when sperm are in the reproductive tract, fertilization can occur.
- Mittelschmerz: Some women feel a dull ache in the lower side of their abdomen, which is a pain known as Mittelschmerz that lasts anywhere between a few minutes to a few hours.
- Duration: The ovulatory phase usually lasts anywhere between 16-32 hours.
What happens during the luteal phase?
- Formation of the corpus luteum and fertilization: If the egg gets fertilized, the ruptured follicle forms a structure called the corpus luteum. This corpus luteum secretes progesterone, which helps implant the fertilized ovum (embryo) in the uterus. Successful implantation of the embryo results in pregnancy.
- Thickening of the endometrium: High estrogen levels during the luteal phase stimulate the endometrium to thicken so that the uterus is strong enough to hold the growing embryo (fetus).
- Degeneration of the corpus luteum: If the egg remains unfertilized or the fertilized egg does not get implanted in the uterus, the corpus luteum degenerates after 14 days. Levels of estrogen and progesterone begin to decrease.
- Duration: The luteal phase begins after ovulation and lasts about 14 days in the absence of fertilization. It ends just before the next menstrual period.