The menstrual cycle is a complex hormonal feedback loop for pregnancy

Science · Published 25 October 2026 Swipe more like this →

In short: The female reproductive system operates through a sophisticated series of recurring hormonal fluctuations. These changes coordinate physical transformations in the uterus and ovaries, preparing the body for potential pregnancy. It is a rhythmic biological process that balances multiple hormones to manage the development of the endometrium and the release of an ovum.

The menstrual cycle typically lasts between 21 and 40 days in adult women, with 28 days being the standard average. While the cycle is often measured starting from the first day of menstruation, menstruation actually represents the conclusion of the previous cycle's processes. This cycle repeats until menopause, which occurs at roughly age 50, when ovarian function ceases. Typical periods last 28 days, yet 21 to 40 days is viewed as standard. This variability means that no woman is always precisely regular in her timing.

The cycle is divided into three primary phases. It begins with approximately five days of menstruation, followed by the proliferative phase lasting until about day 14. During this time, the endometrium—the lining of the uterus—thickens as glands become taller and more vascular. This is followed by the secretory phase, which continues until the next period begins. The physical changes in the endometrium are the external manifestations of these internal hormonal shifts. At the start of the proliferative phase, the endometrium is thin and contains short, straight glands. The ovary remains quiescent during this initial transition. The vascular framework, known as the stroma, separates these tubular glands.

Regulation is driven by a feedback loop involving the pituitary gland and the ovaries. Gonadotropic hormones cause an ovarian follicle to ripen, containing an ovum about 0.14 millimetres in diameter. Within this follicle, granulosa cells multiply and secrete estradiol, a form of estrogen. This hormone is responsible for the proliferative changes in the uterine lining. Other key regulators include progesterone, LH, and FSH, which work in tandem to coordinate the entire reproductive process. The ovum is situated within the wall of the rounded structure formed by the multiplying granulosa cells. This process occurs under the influence of gonadotropic hormones from the pituitary gland. These hormones are essential for the maturation of the follicle.

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