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Eggs, Fertility, and Age: How It Works

Fertility naturally declines as a woman ages, but the decline has little to do with the aging that’s happening to bones, skin, and most of the body’s organs. It’s the age and quality of a woman’s eggs that are the primary reason for the decline in fertility as a woman ages. That’s why women in their 50s and even 60s have been able to carry pregnancies to term...using a younger woman’s donated eggs.

You’ve probably heard that girls are born with a lifetime’s worth of eggs—or pre-eggs—already inside the ovaries. That means that a woman’s eggs are always as old as she is. These one million to two million eggs-in-waiting are called primary oocytes. They’re not really eggs yet, but are encased in a protective cocoon—the follicle—and suspended in a sort of sleep, waiting for the hormones of puberty to awaken them. As time passes, oocytes continuously die. By the time a girl reaches puberty, only 300,000 to 500,000 remain. The rest have disintegrated and been absorbed by the body in a natural process called atresia.

Eggs-traordinary Division
Most cells in the body contain two copies of structures called chromosomes. The chromosomes contain our DNA, our genetic information. Humans have 23 different chromosome pairs, i.e., 46 chromosomes in each cell. When a primary ooctye divides, it goes from having two copies of each chromosome to having only one. Once the oocyte undergoes this cell division, it’s then called an egg. Sperm cells also have just one copy of each chromosome, 23 total. This ensures that when egg and sperm meet and fuse, the resulting fertilized egg—soon to be an embryo—will have two copies of each chromosome in every cell.

This special division process begins at puberty, when the sleeping oocytes that remain awaken. From then on, in every menstrual cycle, “up to one thousand follicles compete to become the dominant one,” explains Michael Thomas, M.D., professor and director of reproductive endocrinology and fertility at the University of Cincinnati Academic Health Center. These eggs undergo cell division, but only one will mature and be released—or ovulated—mid-cycle, perhaps to be fertilized by a sperm cell and begin developing into a baby. (During fertility treatments, when doctors stimulate the ovaries with medication, the goal is to “rescue” some of the follicles that otherwise would have died, allowing multiple follicles—and eggs—to mature.)

Why Eggs Age Poorly
Cell division mistakes can occur during oocyte division. Sometimes a chromosome pair doesn’t separate, and in that case, the egg winds up with 24 chromosomes instead of 23. If that egg is fertilized, the resulting embryo will have one chromosome too many. People with Down Syndrome, for instance, have an extra copy of chromosome No. 21. The chance for mistakes during cell division increase as a woman ages, “and the chances of chromosomal dysfunction or mutation increases,” Dr. Thomas says. This is why the incidence of Down Syndrome and other problems increases with age. Division mistakes and genetic errors in older oocytes may also prevent a fertilized egg from developing properly, leading to miscarriage.

There’s another aspect of egg aging that can pose a problem. Explains Dr. Thomas, “Over time, the covering around the eggs—the zona pellucida—gets thicker, blocking out the sperm. The chance of fertilization decreases.” 

Much of the reason that fertility declines with age has to do with the state of a woman's eggs. But just as some women look younger than their years, women’s eggs decline at different rates, too. There are women in their 30s who have few eggs left, and women in their 40s who are still ovulating regularly. If you’re concerned about the amount and health of your eggs, regardless of age, your doctor can do several tests (blood tests and ultrasounds of the ovaries) to give you an idea of how well your eggs are aging...and how fertile you are.

A version of this article originally published in the Summer 2008 issue of Conceive Magazine.

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