The Female Reproductive Cycle: Stages and Hormonal Control

The Female Reproductive Cycle

The reproductive cycle in girls begins at puberty and ends at menopause, which is when reproductive capacity ceases. In this cycle, two main phases are distinguished: the ovarian cycle and the uterine or menstrual cycle.

The Ovarian Cycle

The ovarian cycle typically lasts 28 days. It develops in two stages and is only interrupted by pregnancy.

• Follicular Growth and Maturation

  This stage is characterized by the maturation of a follicle containing an egg. In the middle of the cycle, the Graafian follicle ruptures on the ovary wall, releasing the egg into the fallopian tube in a process known as ovulation. The oocyte is then conveyed towards the uterus.

• Corpus Luteum Formation

  After ovulation, the ruptured follicle fills with tissue and becomes the corpus luteum. This is reabsorbed by the end of the cycle if no fertilization of the egg has occurred.

The Uterine (Menstrual) Cycle

The first day of the menstrual cycle coincides with the first day of new follicular growth. This cycle consists of a transformation of the inner wall of the uterus, called the endometrium, which occurs in three stages:

• Menstruation

  In the early days of the cycle, there is a flow of blood caused by the rupture of the endometrium and subsequent hemorrhage. The first menstruation a girl experiences is called menarche, and typically, no ovulation occurs on this occasion.

• Proliferative Phase

  After the completion of menstruation and up to approximately 2 days after ovulation (around day 16), the endometrium regenerates.

• Secretory Phase

  Immediately after ovulation, the endometrial tissue becomes hypertrophied and filled with blood vessels. If fertilization occurs, the corpus luteum is maintained on the ovary wall, and the embryo develops within the uterus. Otherwise, between days 27 and 28, the endometrium is shed, and menstruation occurs.

Hormonal Control of the Reproductive Cycle

Three organs produce hormones that control the reproductive cycle: the hypothalamus, pituitary gland, and ovaries. The hypothalamus regulates the production of pituitary hormones, which, in turn, control the secretion of hormones by the ovaries.

• Hormones in the Proliferative Phase

  In the proliferative phase of the ovarian cycle, Follicle-Stimulating Hormone (FSH) is secreted by the pituitary gland. FSH stimulates the maturation of a follicle. The follicle cells produce estrogen, which induces the regeneration of the endometrium after menstruation. At the end of this stage, estrogen inhibits FSH production and stimulates the pituitary gland's secretion of Luteinizing Hormone (LH). When LH levels are high, it triggers ovulation.

• Hormones After Ovulation

  After ovulation, the follicle transforms into the corpus luteum, which secretes progesterone. Progesterone increases the thickness of the endometrium and prevents the development of a new follicle. If no fertilization occurs, progesterone itself inhibits the secretion of LH by the pituitary gland. This triggers the degeneration of the corpus luteum, and consequently, menstruation occurs.