As a woman, you do become less fertile as you get older, eventually culminating in menopause and the end of your potential babymaking years. But what does "less fertile" mean, and at what age, and how quickly does the drop-off in fertility happen?
According to this really fascinating piece by Jean Twenge at The Atlantic, some of the commonly cited scare stats — that one in three women ages 35 to 39 will not be pregnant after a year of trying, say — are based on extremely old data collected from historical birth records that don't necessarily reflect what's happening with real women who are alive right now. That statistic mentioned above, for instance, comes from French records (likely those collected by local church baptismal registries) for the years 1670 to 1830.
That matters because fertility is affected by things like quality of nutrition, infection rates, and even childhood illnesses — all of which have changed drastically for the average Western woman since the 19th century.
Look at more modern records, and the outlook for post-30 babymaking is completely different.
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.
Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.