How Long Can You Wait to Have a Baby?

Sunday, June 30th, 2013

Science reporting is bad, and the popularized statistics on women’s age and fertility are no exception:

The widely cited statistic that one in three women ages 35 to 39 will not be pregnant after a year of trying, for instance, is based on an article published in 2004 in the journal Human Reproduction. Rarely mentioned is the source of the data: French birth records from 1670 to 1830. The chance of remaining childless—30 percent—was also calculated based on historical populations.

In other words, millions of women are being told when to get pregnant based on statistics from a time before electricity, antibiotics, or fertility treatment. Most people assume these numbers are based on large, well-conducted studies of modern women, but they are not. When I mention this to friends and associates, by far the most common reaction is: “No … No way. Really?”

Surprisingly few well-designed studies of female age and natural fertility include women born in the 20th century—but those that do tend to paint a more optimistic picture. One study, published in Obstetrics & Gynecology in 2004 and headed by David Dunson (now of Duke University), examined the chances of pregnancy among 770 European women. It found that with sex at least twice a week, 82 percent of 35-to-39-year-old women conceive within a year, compared with 86 percent of 27-to-34-year-olds. (The fertility of women in their late 20s and early 30s was almost identical—news in and of itself.) Another study, released this March in Fertility and Sterility and led by Kenneth Rothman of Boston University, followed 2,820 Danish women as they tried to get pregnant. Among women having sex during their fertile times, 78 percent of 35-to-40-year-olds got pregnant within a year, compared with 84 percent of 20-to-34-year-olds. A study headed by Anne Steiner, an associate professor at the University of North Carolina School of Medicine, the results of which were presented in June, found that among 38- and 39-year-olds who had been pregnant before, 80 percent of white women of normal weight got pregnant naturally within six months (although that percentage was lower among other races and among the overweight). “In our data, we’re not seeing huge drops until age 40,” she told me.

Even some studies based on historical birth records are more optimistic than what the press normally reports: One found that, in the days before birth control, 89 percent of 38-year-old women were still fertile. Another concluded that the typical woman was able to get pregnant until somewhere between ages 40 and 45. Yet these more encouraging numbers are rarely mentioned—none of these figures appear in the American Society for Reproductive Medicine’s 2008 committee opinion on female age and fertility, which instead relies on the most-ominous historical data.

In short, the “baby panic”—which has by no means abated since it hit me personally—is based largely on questionable data. We’ve rearranged our lives, worried endlessly, and forgone countless career opportunities based on a few statistics about women who resided in thatched-roof huts and never saw a lightbulb. In Dunson’s study of modern women, the difference in pregnancy rates at age 28 versus 37 is only about 4 percentage points. Fertility does decrease with age, but the decline is not steep enough to keep the vast majority of women in their late 30s from having a child. And that, after all, is the whole point.


Studies of natural conception are surprisingly difficult to conduct—that’s one reason both IVF statistics and historical records play an outsize role in fertility reporting. Modern birth records are uninformative, because most women have their children in their 20s and then use birth control or sterilization surgery to prevent pregnancy during their 30s and 40s. Studies asking couples how long it took them to conceive or how long they have been trying to get pregnant are as unreliable as human memory. And finding and studying women who are trying to get pregnant is challenging, as there’s such a narrow window between when they start trying and when some will succeed.

Another problem looms even larger: women who are actively trying to get pregnant at age 35 or later might be less fertile than the average over-35 woman. Some highly fertile women will get pregnant accidentally when they are younger, and others will get pregnant quickly whenever they try, completing their families at a younger age. Those who are left are, disproportionately, the less fertile. Thus, “the observed lower fertility rates among older women presumably overestimate the effect of biological aging,” says Dr. Allen Wilcox, who leads the Reproductive Epidemiology Group at the National Institute of Environmental Health Sciences. “If we’re overestimating the biological decline of fertility with age, this will only be good news to women who have been most fastidious in their birth-control use, and may be more fertile at older ages, on average, than our data would lead them to expect.”


  1. David Foster says:

    Very interesting, but of course the proper metric is not “getting pregnant,” but rather successful pregnancies. Of those 82% of 35 to 39-year-olds who succeeded in getting knocked up within a year, what % actually resulted in a live and healthy birth?

    I think the over-emphasis on “don’t wait too long” is to a considerable extent a reaction to the earlier over-emphasis on “there’s no hurry.”

  2. A very important point to raise, David Foster. My fiancée (a biologist) has expressed to me a strong desire that we should have our children before she reaches age 30, as after that point the incidence of birth defects and miscarriages explodes.

  3. Wobbly says:

    The incidence of Downs goes up a lot after 35, which in most cases ends in an abortion.

  4. KK says:

    Yeah, I wouldn’t be surprised if the study results were interpreted generously in places where it’s convenient. There’s a certain cultural demand for results such as this (although to be fair, that demand exists in the other direction as well).

    The more recent study finds a 0,77 chance for a 35-40 year old woman getting pregnant in the same circumstances as a 20-24 old one. I don’t have access to the paper so I have no idea how the increased likelihood of miscarriages and defects is handled. Their proportion is very significant for the end result, as noted above.

    The similar ratio for men is 0,95. I’m glad to see this effect mentioned out in the open too. Last summer, Greg Cochran did some research on the increased mutational load that an increase in paternal age brings to the table and his results pointed in the same direction. It was news to me back then (although almost obvious in hindsight)

  5. Karen says:

    Speaking of fertility worries, I’m conducting my psychology dissertation research on worry about future fertility among women 25 to 40 years old, and I launched my survey this week! The survey deals with a lot of the issues brought up in this Atlantic article. Please check it out and consider passing it on to your contacts. Thank you!

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