Menstrual Cycle — Core Principles

Core Principles

The menstrual cycle is a monthly physiological process in females, preparing the body for potential pregnancy. It averages 28 days and is divided into ovarian and uterine cycles, both regulated by hormones.

The ovarian cycle includes the follicular phase (follicle growth, estrogen production), ovulation (egg release due to LH surge), and the luteal phase (corpus luteum formation, progesterone production).

The uterine cycle comprises the menstrual phase (shedding of uterine lining), proliferative phase (lining regrowth under estrogen), and secretory phase (lining maturation for implantation under progesterone).

Key hormones are GnRH (hypothalamus), FSH and LH (anterior pituitary), and estrogen and progesterone (ovaries). These hormones operate via intricate positive and negative feedback loops. If fertilization doesn't occur, the corpus luteum degenerates, hormone levels drop, and menstruation begins, restarting the cycle.

Understanding these phases and hormonal interactions is fundamental for NEET aspirants.

Important Differences

vs Estrous Cycle

Aspect	This Topic	Estrous Cycle
Occurrence	Humans, Apes, Old World Monkeys	Most other mammals (e.g., dogs, cats, cows, rats)
Uterine Lining Shedding	Shedding of endometrium occurs (menstruation)	Endometrium is reabsorbed if no pregnancy occurs; no external bleeding
Sexual Receptivity	Sexually receptive throughout the cycle (though fertility varies)	Sexually receptive only during 'estrus' or 'heat' period
Cycle Purpose	Prepares uterus for implantation and releases ovum	Prepares uterus for implantation and releases ovum, but with distinct 'heat' period
Visible Signs	Menstrual bleeding is a visible sign of the cycle's end (or beginning)	No visible bleeding; signs of estrus include behavioral changes, swelling of vulva

The menstrual cycle, characteristic of humans and some primates, involves the cyclical shedding of the uterine lining (menstruation) if pregnancy does not occur, and females are generally receptive to sexual activity throughout the cycle. In contrast, the estrous cycle, found in most other mammals, involves the reabsorption of the uterine lining, not shedding, and females are only sexually receptive during a specific period known as 'estrus' or 'heat', which is marked by distinct behavioral and physiological changes. Both cycles aim to prepare the female reproductive system for pregnancy, but their manifestations differ significantly.