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.