When I was 35, unmarried but still hoping for children someday, I asked my doctor if I should be concerned about my chances of getting pregnant as I got older. Not at all, he assured me: Women are having children into their 40s nowadays. You have plenty of time, he said.

That was more than 10 years ago, just as a boom in "late-timing" motherhood began. Birthrates for U.S. women in their 30s—and 40s and 50s—have increased dramatically since then. (For a rundown on the statistics, see "The Aging of Pregnancy in America.")

Celebrity culture has kept pace featuring pictures of 40-plus actresses, such as Geena Davis and Madonna, cuddling their newborns. The news isn't that middle-aged women are having children—more women had children after 40 in the 1960s. But few of those women were having their first child. For reasons that aren't clear, it is far easier to have your third, or eighth, child after 40 than your first. The real news is that so many women are now having their first child late in life.

My doctor wasn't incorrect in his early '90s optimism about later first-time pregnancy—it does happen more these days. At the same time, the truth is that more older would-be moms are having trouble getting pregnant, more are resorting to expensive, lengthy infertility treatments, and many are eventually discovering that they will not bear their own children after all.

The number of infertility procedures performed between 1996 and 1998 jumped 27 percent. The number of childless women age 40 to 44 has doubled in the past 20 years. A survey conducted by Sylvia Ann Hewlett and described in Creating a Life: Professional Women and the Quest for Children (Talk Miramax, 2002) found that 42 percent of high-achieving women in corporate America were still childless after age 40, a majority of them not by plan. The problem isn't limited to the business world: Recent studies suggest that women in academia may be even more likely to be childless than their corporate sisters.

Though some of these women have clearly made a choice between family and their professions—or are unwilling to undertake parenthood without a partner—most have not made such choices. Rather, they have been caught out by biology. It takes several years to complete professional schooling and to establish a career. But if a woman waits for a suitable time in her career before having children, she may find herself in a race between her own declining fertility and the passage of time.

Assessing your fertility

In the past few years, it has become increasingly clear that reproductive biology harshly separates older women into two groups: women who can get pregnant without a lot of medical intervention, but whose ability to sustain a pregnancy is more compromised than their fertility; and women whose fertility has dipped below a critical line and who require interventions, such as assisted reproductive technology (ART).

For the first group, there is good news: Huge strides have been made in troubleshooting miscarriages, maintaining pregnancies, handling labor complications, and caring for preterm infants—all important issues in late-timing pregnancy.

For the second group, the news is much more grim. According to figures from the Mayo Clinic, a woman's fertility is highest around age 20. Typically, fertility drops 20 percent after age 30, 50 percent after age 35, and 95 percent after age 40. After age 40, reproductive function diminishes drastically: Half of a woman's eggs are chromosomally abnormal at age 40; by 42, that figure is 90 percent.

And should an older woman get pregnant, her chance of miscarriage increases to more than 50 percent by her late 40s. Even the most advanced fertility treatments can't reverse such reproductive decline.

As recently as a decade ago, fertility specialists were much more optimistic about first pregnancies at midlife, given improved overall fitness and advances in reproductive medicine. Now these specialists—and a generation of women—have found instead that certain aspects of fertility are much more intractable than they thought.