The Events of Ovulation

November 08, 2009

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The Events of Ovulation

by Marjorie Greenfield, M.D.
reviewed by Laura Jana, M.D., F.A.A.P.

Ovulation is the process that results in the release of an egg from the ovaries, and represents the time around which a woman is able to become pregnant. It involves a complex interaction between the brain and the ovary. This article introduces the players, and then explains how they interact to cause this extraordinary event.

The players
A region of the brain called the hypothalamus produces the hormone GnRH (gonadotropin releasing hormone). This hormone sends a message to another part of the brain, the pituitary gland, to announce that all is well and that it's OK to have an ovulation cycle.

The pituitary gland is then responsible for signaling the ovaries to release an egg. This signal comes in the form of two hormones: follicle stimulating hormone (FSH) and luteinizing hormone (LH). The pituitary also measures how much estrogen the ovary is making, so it can evaluate the ovary's response to the FSH and LH signals.
Under times of great stress, such as starvation, the hypothalamus does not send the appropriate GnRH signals to the pituitary. This means that a woman is less likely to become pregnant during these times, which makes sense since her body might not be able to offer a nurturing environment for a developing baby.

The ovaries are a pair of glands in the pelvis that make the hormones estrogen and progesterone and carry eggs. All the eggs a woman will ever have are present in her ovaries at birth. (This is in contrast to sperm, which are produced throughout a man's life.)

The events

Early in the ovulation cycle, the pituitary sends a signal telling the ovaries to prepare to release an egg. Many eggs, each in its own bubble called a follicle, start to mature. Each follicle makes estrogen as the egg matures. Eventually one follicle (the "dominant follicle") gets ahead of the others and becomes the only one to continue in the maturation process; all the other eggs die off. Then the dominant follicle puts out a surge of estrogen that tells the pituitary it's ready to ovulate.

Midcycle, when the pituitary gets the estrogen signal from the ovary, it releases a large amount of LH, called the LH surge, which signals the follicle to go ahead and release its egg. Ovulation occurs about 12 to 24 hours after the LH surge, allowing the egg to start making its way down the reproductive tract toward the uterus. The level of LH is what's measured by the ovulation detection kits that you get at the drugstore.

Late cycle. After ovulation, the follicle becomes a corpous luteum, a type of cyst. The corpous luteum is responsible for making progesterone, which is necessary to prepare the uterus to nurture a fertilized egg. Progesterone also causes PMS symptoms like breast soreness, moodiness, and water retention.

Menses or... Once an egg is fertilized and embeds itself in the uterine wall, it begins to produce human Chorionic Gonadotropin (hCG). This is the hormone that a pregnancy test measures. The presence of hCG tells the ovary to continue to make progesterone. If the ovary doesn't receive this hormonal signal, it stops making progesterone 9 to 11 days after ovulation, and as the levels of progesterone fall, the uterine lining is shed (menstruation). This drop in hormone levels and shedding of the uterine lining in turn launches another ovulation cycle.

Cycle of events can vary
The length of the early part of the menstrual cycle, from the first day of the period to ovulation, is quite variable. In some women, it's as short as eight or nine days; in others, it might be as long as 21 days or more. Women with irregular cycles may be variable month to month.

The late part of the cycle, from the time an egg is released from the ovary to the start of menstruation, is exactly 14 days in all women. That's how long the corpous luteum can make progesterone while waiting to see if an egg has been fertilized and a new life begun.


RELATED INFORMATION

			Birth Control: An Overview
			Trying to Conceive: Myths and Truths
			Birth Control
			Getting Pregnant
			Menstrual Issues
			Infertility

Created March 16, 2001
Reviewed March 22, 2001

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