The best way to assess fertility might be to measure “cycle viability,” or the chance of getting pregnant if a couple has sex on the most fertile day of the woman’s cycle. Studies based on cycle viability use a prospective rather than retrospective design—monitoring couples as they attempt to get pregnant instead of asking couples to recall how long it took them to get pregnant or how long they tried. Cycle-viability studies also eliminate the need to account for older couples’ less active sex lives. David Dunson’s analysis revealed that intercourse two days before ovulation resulted in pregnancy 29 percent of the time for 35-to-39-year-old women, compared with about 42 percent for 27-to-29-year-olds. So, by this measure, fertility falls by about a third from a woman’s late 20s to her late 30s. However, a 35-to-39-year-old’s fertility two days before ovulation was the same as a 19-to-26-year-old’s fertility three days before ovulation: according to Dunson’s data, older couples who time sex just one day better than younger ones will effectively eliminate the age difference.
Don’t these numbers contradict the statistics you sometimes see in the popular press that only 20 percent of 30-year-old women and 5 percent of 40-year-old women get pregnant per cycle? They do, but no journal article I could locate contained these numbers, and none of the experts I contacted could tell me what data set they were based on. The American Society for Reproductive Medicine’s guide provides no citation for these statistics; when I contacted the association’s press office asking where they came from, a representative said they were simplified for a popular audience, and did not provide a specific citation.
Dunson, a biostatistics professor, thought the lower numbers might be averages across many cycles rather than the chances of getting pregnant during the first cycle of trying. More women will get pregnant during the first cycle than in each subsequent one because the most fertile will conceive quickly, and those left will have lower fertility on average.
Most fertility problems are not the result of female age. Blocked tubes and endometriosis (a condition in which the cells lining the uterus also grow outside it) strike both younger and older women. Almost half of infertility problems trace back to the man, and these seem to be more common among older men, although research suggests that men’s fertility declines only gradually with age.
Fertility problems unrelated to female age may also explain why, in many studies, fertility at older ages is considerably higher among women who have been pregnant before. Among couples who haven’t had an accidental pregnancy—who, as Dr. Steiner put it, “have never had an ‘oops’ ”—sperm issues and blocked tubes may be more likely. Thus, the data from women who already have a child may give a more accurate picture of the fertility decline due to “ovarian aging.” In Kenneth Rothman’s study of the Danish women, among those who’d given birth at least once previously, the chance of getting pregnant at age 40 was similar to that at age 20.
Older women’s fears, of course, extend beyond the ability to get pregnant. The rates of miscarriages and birth defects rise with age, and worries over both have been well ventilated in the popular press. But how much do these risks actually rise? Many miscarriage statistics come from—you guessed it—women who undergo IVF or other fertility treatment, who may have a higher miscarriage risk regardless of age. Nonetheless, the National Vital Statistics Reports, which draw data from the general population, find that 15 percent of women ages 20 to 34, 27 percent of women 35 to 39, and 26 percent of women 40 to 44 report having had a miscarriage. These increases are hardly insignificant, and the true rate of miscarriages is higher, since many miscarriages occur extremely early in a pregnancy—before a missed period or pregnancy test. Yet it should be noted that even for older women, the likelihood of a pregnancy’s continuing is nearly three times that of having a known miscarriage.
What about birth defects? The risk of chromosomal abnormalities such as Down syndrome does rise with a woman’s age—such abnormalities are the source of many of those very early, undetected miscarriages. However, the probability of having a child with a chromosomal abnormality remains extremely low. Even at early fetal testing (known as chorionic villus sampling), 99 percent of fetuses are chromosomally normal among 35-year-old pregnant women, and 97 percent among 40-year-olds. At 45, when most women can no longer get pregnant, 87 percent of fetuses are still normal. (Many of those that are not will later be miscarried.) In the near future, fetal genetic testing will be done with a simple blood test, making it even easier than it is today for women to get early information about possible genetic issues.
What does all this mean for a woman trying to decide when to have children? More specifically, how long can she safely wait?
This question can’t be answered with absolutely certainty, for two big reasons. First, while the data on natural fertility among modern women are proliferating, they are still sparse. Collectively, the three modern studies by Dunson, Rothman, and Steiner included only about 400 women 35 or older, and they might not be representative of all such women trying to conceive.
Second, statistics, of course, can tell us only about probabilities and averages—they offer no guarantees to any particular person. “Even if we had good estimates for the average biological decline in fertility with age, that is still of relatively limited use to individuals, given the large range of fertility found in healthy women,” says Allen Wilcox of the NIH.
So what is a woman—and her partner—to do?
The data, imperfect as they are, suggest two conclusions. No. 1: fertility declines with age. No. 2, and much more relevant: the vast majority of women in their late 30s will be able to get pregnant on their own. The bottom line for women, in my view, is: plan to have your last child by the time you turn 40. Beyond that, you’re rolling the dice, though they may still come up in your favor. “Fertility is relatively stable until the late 30s, with the inflection point somewhere around 38 or 39,” Steiner told me. “Women in their early 30s can think about years, but in their late 30s, they need to be thinking about months.” That’s also why many experts advise that women older than 35 should see a fertility specialist if they haven’t conceived after six months—particularly if it’s been six months of sex during fertile times.
There is no single best time to have a child. Some women and couples will find that starting—and finishing—their families in their 20s is what’s best for them, all things considered. They just shouldn’t let alarmist rhetoric push them to become parents before they’re ready. Having children at a young age slightly lowers the risks of infertility and chromosomal abnormalities, and moderately lowers the risk of miscarriage. But it also carries costs for relationships and careers. Literally: an analysis by one economist found that, on average, every year a woman postpones having children leads to a 10 percent increase in career earnings.
For women who aren’t ready for children in their early 30s but are still worried about waiting, new technologies—albeit imperfect ones—offer a third option. Some women choose to freeze their eggs, having a fertility doctor extract eggs when they are still young (say, early 30s) and cryogenically preserve them. Then, if they haven’t had children by their self-imposed deadline, they can thaw the eggs, fertilize them, and implant the embryos using IVF. Because the eggs will be younger, success rates are theoretically higher. The downsides are the expense—perhaps $10,000 for the egg freezing and an average of more than $12,000 per cycle for IVF—and having to use IVF to get pregnant. Women who already have a partner can, alternatively, freeze embryos, a more common procedure that also uses IVF technology.
At home, couples should recognize that having sex at the most fertile time of the cycle matters enormously, potentially making the difference between an easy conception in the bedroom and expensive fertility treatment in a clinic. Rothman’s study found that timing sex around ovulation narrowed the fertility gap between younger and older women. Women older than 35 who want to get pregnant should consider recapturing the glory of their 20‑something sex lives, or learning to predict ovulation by charting their cycles or using a fertility monitor.
I wish I had known all this back in the spring of 2002, when the media coverage of age and infertility was deafening. I did, though, find some relief from the smart women of Saturday Night Live.
“According to author Sylvia Hewlett, career women shouldn’t wait to have babies, because our fertility takes a steep drop-off after age 27,” Tina Fey said during a “Weekend Update” sketch. “And Sylvia’s right; I definitely should have had a baby when I was 27, living in Chicago over a biker bar, pulling down a cool $12,000 a year. That would have worked out great.” Rachel Dratch said, “Yeah. Sylvia, um, thanks for reminding me that I have to hurry up and have a baby. Uh, me and my four cats will get right on that.”
“My neighbor has this adorable, cute little Chinese baby that speaks Italian,” noted Amy Poehler. “So, you know, I’ll just buy one of those.” Maya Rudolph rounded out the rant: “Yeah, Sylvia, maybe your next book should tell men our age to stop playing Grand Theft Auto III and holding out for the chick from Alias.” (“You’re not gonna get the chick from Alias,” Fey advised.)
Eleven years later, these four women have eight children among them, all but one born when they were older than 35. It’s good to be right.