Hormone Cycle

Learn about the hormone cycle in women

Women are born with all the eggs in their ovaries that they will need for a reproductive lifetime. Each month, as part of the menstrual cycle, follicles develop on the ovaries. Each follicle contains one egg and its development is stimulated and supported by FSH which is released by the pituitary gland under the influence of the hypothalamus.

Female menstrual cycle, ovulation process and hormone levels, detailed medical illustration.

Female menstrual cycle, ovulation process and hormone levels, detailed medical illustration.

The female reproductive processes involve complex interactions between several hormones ultimately leading to follicle recruitment, egg development, ovulation, fertilization and implantation. The hypothalamus can be thought of as the “reproductive hormone control center” functioning like a thermostat as it adjusts various hormone levels. During the first few days of the ovulatory cycle, the hypothalamus produces gonadotropin releasing hormone (GnRH), which stimulates the pituitary to produce follicle stimulating hormone (FSH & LH). FSH causes follicular recruitment (each follicle contains one egg) and supports the follicles as they mature.

As healthy follicles mature they produce estrogen, which stimulates the development of the endometrium (lining of the uterus). The hypothalamus monitors these levels and, as they increase, reduces the production of GnRH thus lowering FSH production. Once the follicles mature, the hypothalamus signals the pituitary to release a surge of luteinizing hormone (LH) that triggers ovulation 36 hours later.

After ovulation, the remaining follicular structure forms the corpus luteum, which begins production of progesterone. Once the placenta develops, it also begins to produce progesterone to support the developing fetus and the corpus luteum digresses.