Menstruation, also known as the menstrual cycle, is a monthly process that occurs in the female reproductive system. It involves the shedding of the uterine lining (endometrium) when fertilization does not occur. This process is regulated by complex hormonal interactions and is essential for fertility.

The menstrual cycle typically lasts about 28 days, though it can range from 21 to 35 days. The cycle is divided into distinct phases: the follicular phase, ovulation, the luteal phase, and menstruation.

Hormonal Regulation of the Menstrual Cycle

The menstrual cycle is controlled by hormones secreted by the hypothalamus, pituitary gland, and ovaries. The primary hormones involved are:

1. Gonadotropin-Releasing Hormone (GnRH):
   • Secreted by the hypothalamus.
   • Stimulates the release of Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) from the anterior pituitary gland.
2. Follicle-Stimulating Hormone (FSH):
   • Released by the pituitary gland.
   • Stimulates the growth and maturation of ovarian follicles (each containing an egg).
3. Luteinizing Hormone (LH):
   • Also released by the pituitary gland.
   • Triggers ovulation, the release of a mature egg from the ovary.
4. Estrogen:
   • Produced by developing ovarian follicles and later by the corpus luteum.
   • Responsible for the growth and maturation of the uterine lining (endometrium) in preparation for pregnancy.
5. Progesterone:
   • Produced by the corpus luteum after ovulation.
   • Helps maintain the uterine lining and prepares it for potential implantation of a fertilized egg.
6. Prostaglandins:
   • Produced in the endometrium.
   • Play a role in the contraction of uterine muscles and the shedding of the endometrial lining during menstruation.

Phases of the Menstrual Cycle

The menstrual cycle can be divided into four main phases:

1. Menstrual Phase (Day 1–5)

• Menstruation (the shedding of the endometrial lining) begins if the egg has not been fertilized.
• Hormonal Changes: The levels of estrogen and progesterone decrease, leading to the breakdown of the endometrial lining.
• Physical Manifestation: The lining of the uterus is shed through the vagina, resulting in menstrual bleeding.

2. Follicular Phase (Day 1–14)

• Follicle Development: FSH stimulates several follicles in the ovaries to mature, each containing an immature egg.
• Estrogen Production: As the follicles mature, they release estrogen, which stimulates the growth and thickening of the endometrium in preparation for a potential pregnancy.
• Hormonal Changes: Rising estrogen levels signal the pituitary gland to decrease FSH and increase LH. The highest estrogen levels occur just before ovulation.
• Physical Manifestation: As estrogen increases, the lining of the uterus becomes thicker and more vascular.

3. Ovulation (Day 14)

• Release of the Egg: A peak in LH levels triggers ovulation, where a mature follicle releases an egg into the fallopian tube.
• Hormonal Changes: Estrogen levels peak just before ovulation, followed by a surge in LH, which causes the egg to be released from the ovary.
• Physical Manifestation: The egg travels down the fallopian tube, where it may be fertilized by sperm. This is the window of fertility in the menstrual cycle.

4. Luteal Phase (Day 15–28)

• Formation of Corpus Luteum: After ovulation, the ruptured follicle transforms into the corpus luteum, which secretes progesterone.
• Progesterone Production: Progesterone prepares the endometrium for implantation by thickening it further and increasing its blood supply.
• Hormonal Changes: If the egg is fertilized and implantation occurs, human chorionic gonadotropin (hCG) is produced to maintain the corpus luteum. If pregnancy does not occur, the corpus luteum degenerates, leading to a decrease in progesterone.
• Physical Manifestation: The uterus remains ready to support a potential pregnancy. If no pregnancy occurs, the endometrium starts to break down, leading to the start of the next menstrual phase.

Endometrial Changes During the Menstrual Cycle

1. Menstrual Phase (Day 1–5): The endometrium sheds its superficial layer, resulting in menstrual bleeding.
2. Proliferative Phase (Part of Follicular Phase): Estrogen stimulates the growth of the endometrial lining.
3. Secretory Phase (Part of Luteal Phase): Progesterone increases the vascularity and thickness of the endometrial lining, preparing it for embryo implantation.
4. Ischemic Phase: If pregnancy does not occur, the corpus luteum degenerates, causing a drop in progesterone, leading to the constriction of blood vessels in the endometrium. This results in the shedding of the endometrial lining.

Menstruation and Pregnancy

• Pregnancy: If sperm fertilizes the egg during ovulation, the fertilized egg (zygote) will travel to the uterus and implant in the endometrial lining. This triggers the production of hCG, which maintains the corpus luteum and prevents the shedding of the uterine lining. As a result, menstruation does not occur.
• No Pregnancy: If fertilization does not occur, the corpus luteum degenerates, progesterone levels fall, and the endometrial lining breaks down. The shedding of the lining is what is observed as menstruation.

Regulation of Menstruation

The menstrual cycle is regulated by a feedback loop between the hypothalamus, pituitary gland, and ovaries:

• Hypothalamus releases GnRH, which stimulates the pituitary gland to release FSH and LH.
• FSH and LH control the maturation of follicles and ovulation.
• Estrogen and progesterone, produced by the ovaries, regulate the growth of the endometrium and feedback to the hypothalamus and pituitary gland to modulate their function.

Disorders of Menstruation

Several conditions can affect menstruation, including:

• Amenorrhea: Absence of menstruation, which can be primary (never having a period) or secondary (absence of periods for at least three months).
• Dysmenorrhea: Painful menstruation, often caused by excessive prostaglandin production leading to uterine contractions.
• Menorrhagia: Heavy or prolonged menstrual bleeding.
• Oligomenorrhea: Irregular or infrequent menstruation.
• Premenstrual Syndrome (PMS): Symptoms like mood swings, irritability, fatigue, and bloating that occur before menstruation.

Conclusion

Menstruation is a complex physiological process regulated by hormonal signals from the brain and ovaries. It involves the maturation and release of an egg, the preparation of the uterine lining, and the shedding of that lining if fertilization does not occur. The menstrual cycle is crucial for female fertility and is a dynamic process that is influenced by various factors, including hormonal balance, health, and lifestyle.