The ovaries are small, oval glands located on either side of the uterus. They produce and store eggs and produce hormones that control the menstrual cycle and pregnancy. During ovulation, one of the ovaries releases an egg. If a sperm fertilizes this egg, pregnancy occurs. The ovaries continue to release an egg each menstrual cycle until menopause occurs. During menopause, the ovaries stop producing eggs. Sometimes the ovaries can release more than one egg (this can lead to a multiple pregnancy). Every woman is born with all the eggs she will ever have in her lifetime. The ovaries play a major role in both menstruation and conception. They produce eggs needed for fertilization and produce the hormones estrogen and progesterone. The ovary releases an egg around the middle of the menstrual cycle (around day 14 of the 28-day cycle) in a process called ovulation. Each of the ovaries has thousands of ovarian follicles. Ovarian follicles are small sacs in the ovaries that contain immature eggs. Each month, between the sixth and the 14th day of the menstrual cycle, follicle-stimulating hormone causes the follicles in one of the ovaries to mature. Around day 14 of the menstrual cycle, a sudden spike in luteinizing hormone levels causes the ovary to release an egg (ovulation). The egg begins its journey through a narrow, hollow structure called a fallopian tube to the uterus. As the egg travels through the fallopian tube, the level of progesterone rises, which helps prepare the lining of the uterus for pregnancy. The ovaries produce estrogen and progesterone. These hormones play an important role in reproductive development and menstruation. Estrogen production is highest during the first half of the menstrual cycle before ovulation. Progesterone rises during the second half of the cycle to prepare the uterus for a fertilized egg if conception occurs. The empty follicle left in the ovary after ovulation is known as the corpus luteum. This structure releases the hormones estrogen (in a lower amount) and progesterone (in a higher amount). These hormones then prepare the uterine lining for possible pregnancy – in the event that the recently released egg is fertilized. If the released egg is not fertilized, the corpus luteum disintegrates, stopping the secretion of estrogen and progesterone. Now that these hormones are no longer present, the lining of the uterus begins to break down and is shed from the body through menstruation. Then another cycle starts again. If the released egg is fertilized, the corpus luteum remains functional and continues to secrete estrogen and progesterone; This allows the embryo to implant in the lining of the uterus and form a placenta. From here, a fetus begins to develop. Despite profound changes in the number and quality of follicles during the third and fourth decades of life,the ovarian aging process remains largely unnoticed. Events that can be quite easily recognized are irregular periods and menopause. The onset of declining fertility and the subsequent loss of natural fertility—the lost ability to create a viable ongoing pregnancy leading to the birth of a child—cannot be directly recognized on an individual basis. The first subtle clinical sign of progress in the reproductive aging process is a shortening of the length of the menstrual cycle by 2 or 3 days. It is only when menstrual cycles become irregular that women usually first notice the signs of the continued decline in the number of follicles. The insufficient availability of follicles leads to an increase in the frequency of prolonged cycles or missed periods and even prolonged intervals of stopping the cycle. References: 1. Society for Endocrinology. Ovaries (https://www.yourhormones.info/glands/ovaries/). 2. The Pituitary Foundation. Your hormones (https://www.pituitary.org.uk/information/hormones/). 3. National Cancer Institute. Ovarian follicle (https://www.cancer.gov/publications/dictionaries/cancer-terms/def/ovarian-follicle) & ovary (https://www.cancer.gov/publications/dictionaries/cancer-terms/def/ ovary). 4. Williams gynecology. Hoffman, Barbara L., Williams, J. Whitridge (John Whitridge), 1866-1931. (2nd ed.). New York: McGraw-Hill Medical.