Gynecologic Health Gynecologic Health Basics

Checking Your Biological Clock

By Christine Haran

Despite the long lifespan American women now enjoy, their reproductive lifespan remains unchanged. In most women, a gradual decline in fertility begins in the late 20s and then drops off more sharply around age 35 and then again around 40. Yet fertility in this age group still varies from woman to woman.

That's one reason why the idea of a test assessing a woman's ovarian reserve—which depends upon the number and quality of the eggs in the ovaries and how the ovaries respond to certain hormones—is appealing for women seeking fertility treatment, and maybe even for those women who just want a better idea of how fast their biological clock is ticking. A study published in the June 17th issue of the journal Human Reproduction found that ovarian reserve may be determined through an ultrasound, sparking interest in the utility of assessing one's "reproductive age."

Egg Timers
Though it seems hard to believe, the biological clock begins ticking before female babies are even born. After about 20 weeks in the womb, female fetuses have about seven million eggs, but by the time they're born, they're down to about one to two million. Over the next 35 to 40 years, women ovulate 300 to 400 eggs and the rest gradually die on their own until a woman goes into menopause around age 50.

According to the American Society for Reproductive Medicine, a healthy 30-year-old woman has about a 20 percent chance per month of becoming pregnant, while a 40-year-old woman has about a 5 percent chance per month. Although age is the most powerful predictor of fertility, other factors play a role in how quickly one's ovarian reserve diminishes.

"If you take two 35-year-old women, you have one that may go through this process significantly faster than another 35-year-old," says Fady Sharara, MD, medical director for the Virginia Center for Reproductive Medicine and an associate clinical professor at George Washington University in Washington, DC. "There are some factors that can contribute to this, such as genetic background, smoking, ovarian surgery and exposure to radiation."

Assessing Ovarian Reserve
In order to achieve pregnancy, the follicle-stimulating hormone (FSH) must trigger the development of follicle cells in the ovaries that contain immature eggs. One of these eggs will mature in a process known as ovulation and travel though the fallopian tube to the uterus. If the egg is fertilized by a sperm cell at this time, it may attach itself to the uterine wall and pregnancy may occur.

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