It’s a basic biological fact: At birth, a newborn girl’s body contains all the eggs she’ll ever have—literally over a million “potential futures.” But starting from day one, these immature eggs begin to self-destruct and be absorbed by the body in a natural process called “atresia.” By the time that baby girl reaches puberty, only 300,000 eggs will remain in her ovaries. And over the course of her reproductive years, only about 300 of these will actually mature and be released, one each month for approximately 25 years. Unlike men, who continue to produce newly-minted sperm their entire lives, women are ruled by the numbers.
“In essence, a woman’s reproductive tract has a timeline of its own, independent of the rest of her body’s aging,” explains Michael R. Soules, MD, professor and director of reproductive endocrinology and infertility, department of obstetrics/gynecology, University of Washington, Seattle. And despite headlines that suggest that “sixty is the new thirty,” when it comes to a woman’s eggs and reproductive capacity, that just isn’t true.
Body Basics and “That Clock”
Beginning roughly around the time of her first period, a woman’s ovaries begin the business of preparing eggs for possible fertilization. Each month the most robust—or “dominant”—egg is ripened and released by one of the ovaries. The egg then wends its way into the fallopian tube to await possible fertilization. The peak years for egg maturation and release are during a woman’s mid- to late-twenties. Before that, the menstrual cycle may not be precisely synchronized, with a somewhat greater chance of so-called anovulatory cycles, in which menstruation occurs but no eggs are released.
When a woman reaches her thirties, she usually still has plenty of eggs left in her ovaries, but starting sometime around the middle of that decade, the follicles that surround and release the eggs may become less responsive to hormonal signals. At the same time, atresia— the slow, steady destruction of “pre-eggs”—continues, regardless of whether a woman continues to menstruate, becomes pregnant, uses contraception, or undergoes fertility treatments. It is a process as inevitable as time—and is, in fact, the true determination of a woman’s so-called “biological clock.”
Every woman’s “clock” is set to a very individualized timer. Some women, for unknown reasons, exhaust all their viable eggs early and are unable to conceive even while still young. Other women have no trouble whatsoever becoming pregnant and having healthy babies in their late thirties or even forties. The problem: Currently there’s no reliable way of determining what any individual woman’s timer is set to.
“The tests we have right now are pretty good at determining what we call a woman’s ‘ovarian reserve,’ or number and responsiveness of eggs, for the next six months, but beyond that we can’t really say,” admits James Liu, MD, chair of the department of obstetrics and gynecology/reproductive biology at University Women’s Macdonald Hospital, Cleveland, Ohio.
Women can get a few clues about their future by looking at their mothers and grandmothers. A family history of late menopause (the average is age 48 to 52) may mean a woman will also be likely to have a somewhat longer stretch of fertility, but it’s no guarantee. “It might translate into a two-to-three year advantage, but there’s no way to know for sure,” says Dr. Liu. In fact, at a special workshop on Stages of Reproductive Aging hosted in 2001 by the National Institutes of Health, top experts agreed that, unlike puberty and menopause, fertility couldn’t be broken down into precise stages. “Chronological age is a very poor indicator of reproductive aging,” the panelists agreed, noting that studies that do exist suggest individual women may be born with “a highly variable number of oocytes [eggs], or the rate of oocyte loss varies greatly.”
Ovarian Reserve What/When to Test
When a couple is unable to conceive, it’s reasonable to wonder if the problem lies with a woman’s eggs. It used to be the rule that fertility experts wouldn’t do tests on a couple until they had been trying to conceive for a full year without success, and many insurance companies wouldn’t cover any tests or treatments until after two years. But more sophisticated tests and treatments now mean that when it comes to assessing ovarian reserve, many doctors are urging women not to wait that long.
Nanette Santoro, MD, professor and director of reproductive endocrinology and infertility at Albert Einstein College of Medicine in New York City, advises women over 35 to go for testing after six months of trying to get pregnant, and takes an even more aggressive approach with younger women, who, theoretically, should be able to conceive even faster. “Women who are at peak reproductive age (under 30) who have irregular periods or changes in their menstrual lengths should have an assessment made by their gynecologist after three months of trying but not succeeding to become pregnant, in my opinion,” she says. “Women 30 to 35 should be considered for ovarian reserve testing within six months of attempting pregnancy unsuccessfully, or sooner if they are menstruating irregularly,” she says.
The Key “Egg Tests” to Have
Clomiphene Citrate Challenge Test (CCCT)
This test entails measuring blood levels of FSH (follicle-stimulating hormone) on day three of the menstrual cycle, then taking 100 mg of the ovary-stimulating drug clomiphene citrate on menstrual cycle days five-to-nine. Blood levels of FSH are then measured again on cycle day 10. Many doctors like to repeat the test to rule out a one-cycle aberration. Elevated FSH levels (over 10) are generally considered to indicate low-responsiveness, as the body is thought to be over-releasing the hormone in an effort to “jump start” a sluggish reaction.
Measuring Circulating Levels of Inhibin-B
This blood test assesses the level of a chemical produced by the granulosa cells of the ovaries that signals the pituitary gland to slow down secretion of FSH.
This test uses ultrasound to assess the numbers of “antral follicles” — those follicles ready to produce and release a mature egg. “In this case you want ten as a minimum,” says Dr. Soules. “A result of six to ten is considered low but amenable to treatment with ovulation- inducing drugs; below that, the only real option would be donor eggs.”
Taking Your Own Time to Get Pregnant
Whatever a woman’s own health history or age, the state of her life is as important as the state of her eggs. “It’s not realistic for us to suggest that every woman in America should try to have her children during her twenties because her eggs are most plentiful and it’s biologically easier,” says Dr. Soules. “Other factors, ranging from meeting the right man to establishing a career to simply earning enough money to support a family come into play.”
And even scientific statistics don’t tell the whole story. “The effect of age on reproductive efficiency—or how likely a woman is to get pregnant if she has regular sexual intercourse—is relatively slight up to age 30, becomes measurable between ages 30 to 35 (but is still not huge), and rises drastically between ages 35 and 40,” says Dr. Santoro. That said, the fastest growing group of mothers in this country are those between ages 35 to 39. While some of those women are not having their first child (a “track record of having a healthy child with the same partner means a more likely chance of success than an ‘untested system,’” says Dr. Liu), many are becoming first-time mothers without any “outside assistance.”
Statistics from the American Society of Reproductive Medicine show that two-thirds of couples in which the woman is over 35 have no difficulty conceiving. “We don’t hear about the success stories because they don’t make for dramatic headlines,” notes Dr. Soules. But given the reality that stress can also have an adverse effect on conception, it may make sense to relax. The “payoff” could be a new, healthy life.
A version of this article originally appeared in the Premier issue of Conceive Magazine.
Related Topics: Age and Fertility; Fertility Basics; Fertility Tips