Getting pregnant after age 35 might not be the freak miracle of nature news weeklies and sitcoms would have you believe. Someone more qualified to parse fertility studies than your average journalist (and who happened to have three children after 35) went back to look at the data and found the baby panic "is based largely on questionable data." Writing in the Atlantic, San Diego State University psychology professor Jean Twenge says the widely cited figure that one in three women ages 35 to 39 will not be pregnant after a year of trying is based on French birth records from 1670 to 1830. "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," she writes.
Twenge blames the "availability heuristic" for perpetuating the myth of the fast-ticking biological clock. When talking about infertility, we tend to talk to fertility specialists, who see the small percentage of people using in vitro fertilization, and not the millions of people easily procreating. Plus, the women who are trying to get pregnant after 35 may suffer from non-age-related fertility problems (blocked tubes, weak sperm), which explains why they haven’t had an accidental pregnancy already. According to a contemporary study of women trying to get pregnant, fertility doesn’t meaningfully drop off until after 40. In a study of Danish 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."
But while the rate at which fertility decreases with age may be overblown, the degree to which child-bearing affects your earning potential is not. Twenge writes, "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." To paraphrase Sheryl Sandberg, don’t leave before you're 40.