In addition to producing eggs, the ovaries also secrete hormones. Hormones are substances secreted from organs of the body, such as the pituitary gland, adrenal gland, or ovaries, which are carried by a bodily fluid such as blood to other organs or tissues where the substances exert a specific action. The cycle of ovarian hormone production has two main phases. In the first phase, known as the follicular phase, an egg matures inside the ovary.

The egg is surrounded by a layer of hormone-producing cells and fluid. The maturing egg, the surrounding cells, and the fluid are collectively known as a follicle.

The follicle grows to a diameter of about an inch, forming a cyst-like sac on the surface of the ovary, before the fluid and the egg are released at ovulation.

In natural cycles, an ovary contains several developing follicles, but usually only one follicle reaches maturity each month and releases an egg.

This follicle, known as the dominant follicle, secretes a generous amount of the female hormone estradiol (estrogen) into the bloodstream during the first phase of the cycle. The estrogen circulates to the uterus where it stimulates the endometrial cells to reproduce rapidly and repeatedly, causing the uterine lining to thicken as ovulation approaches. The fertility specialist can usually see this thickening on an ultrasound exam.

The second phase of ovarian hormone production begins with ovulation. The dominant follicle ruptures, usually around day 14 in a 28-day cycle, and releases a mature egg onto the surface of the ovary near the fallopian tube. The empty follicle collapses and the remaining follicle cells develop a yellow color. Collectively these cells are known as the corpus luteum, literally a “yellow body.” The corpus luteum secretes estrogen and large quantities of progesterone throughout the second half of the cycle, known as the luteal phase, which lasts approximately two weeks.

Traveling through the bloodstream to the uterus, the combination of progesterone and estrogen causes the uterine lining to further mature and produce nourishment for an embryo. About a week after ovulation, the endometrium is in prime condition for an embryo to implant. An experienced fertility specialist can tell approximately how many days have passed since ovulation by examining a sample of the endometrium taken in a biopsy. If no embryo implants, the secretion of estrogen and progesterone declines about two weeks after ovulation and, as a result, the endometrium is shed. This shedding of the endometrium is called menstruation.

The first day of menstruation is known as “cycle day one.” The length of the menstrual cycle is determined by counting the number of days from cycle day one until the start of the next menstrual period. Although variability in cycle length is usually due to variability in the follicular phase, the luteal phase can also be variable in length. The luteal phase should last 11 to 16 days.

If it is not sufficient in length because of inadequate progesterone production, fertility problems may result. Since ovulation usually precedes menstruation by two weeks, a woman with a 28-day menstrual cycle is most likely to ovulate on day 14. Similarly, a woman with a 32-day cycle is most likely to ovulate on day 18.

Directives From the Brain

The hypothalamus and pituitary gland orchestrate the events leading to ovulation. These organs communicate with the ovaries via hormonal messengers traveling in the bloodstream. The hypothalamus is a thumb-sized structure in the base of the brain that controls many bodily functions and regulates the pituitary gland. The pituitary gland, about the size of a finger tip, is located just beneath the hypothalamus.

The hypothalamus releases the hormone gonadotropin releasing hormone (GnRH), a messenger that tells the pituitary gland to release follicle stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH are both involved in maturing the follicle, but FSH primarily makes the follicle grow and produce increasing amounts of estrogen. As the follicle grows, the increasing estrogen in the bloodstream signals the pituitary to shut down FSH production. A surge of LH triggers ovulation. After ovulation, the corpus luteum primarily produces progesterone, which prepares the endometrium for the implantation of a fertilized egg.