Updated by: Arabinda Naik
The menstrual cycle is a series of cyclic changes that occur in the female reproductive system to prepare for possible pregnancy. It involves hormonal regulation and structural changes in the ovaries and uterus. The cycle repeats approximately every 28 days and continues from puberty (12-15 years) to menopause (45-55 years).
Menarche: The first menstrual cycle, occurring at puberty.
Menopause: The natural cessation of menstrual cycles, marking the end of reproductive capability.
Gonadotropins: Hormones (LH and FSH) that regulate the cycle.
Ovulation: The release of a mature ovum from the ovary.
Oestrogen and Progesterone: Hormones that control changes in the uterus during the cycle.
The menstrual phase is the first phase of the menstrual cycle, marking the beginning of a new cycle in the female reproductive system. It is also referred to as the bleeding phase or menses and occurs only if fertilization does not take place. The phase typically lasts 3 to 7 days, but on average, it lasts 4–5 days in most women. The cycle starts from Day 1 of menstruation.
The menstrual phase occurs only when fertilization does not happen. The corpus luteum degenerates, leading to a drop in the levels of progesterone and estrogen. This hormonal decline causes the breakdown of the endometrial lining of the uterus. The endometrial lining of the uterus (which was prepared for implantation) sheds off. Blood, tissue fluid, mucus, and the unfertilized ovum are discharged from the uterus through the vagina. This shedding is known as menstrual flow or menstruation. The average blood loss is around 45–100 mL.
The menstrual blood does not clot due to the presence of an enzyme called fibrinolysin.
The levels of Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH) are low during this phase. The withdrawal of LH leads to the degeneration of the corpus luteum, stopping progesterone production. The decrease in estrogen and progesterone triggers the breakdown of the endometrial lining. Women may experience abdominal cramps, fatigue, mood swings, nausea, headaches, and bloating. Prostaglandins, which are released to help expel the endometrial lining, can cause uterine contractions, leading to cramps. The menstrual phase marks the beginning of a new reproductive cycle. If pregnancy does not occur, the uterus prepares for another cycle of ovulation and potential fertilization.