What's the "best age" to have a baby? That depends, in part, on what you mean by "best."

The late teens or early twenties are best biologically, according to John Mirowsky, a sociologist at the University of Texas at Austin. That’s when “oocytes are fresh and the body’s reproductive and other systems are at a youthful peak,” he wrote. Women in their twenties are least likely to have developed chronic health problems that would put them or their babies at risk, and they have the lowest rates of miscarriage, ectopic pregnancy, stillbirth, and infertility. But early pregnancy doesn’t work well in today’s society, which is organized around smaller families and more full-time employment for women. If pregnancy occurs too early, social difficulties often follow. At age 20 or younger, Mirowsky wrote, pregnancy is “more likely to happen out of wedlock, more likely to interfere with educational attainment, and more likely to crystallize a disadvantaged status.”

Even the early twenties seems too young, in some circles, to have a baby— even when the mother is married and has a college degree. Michelle Horton, for instance, gave birth to her son, Noah, when she was 21, an age that seemed reasonable enough a generation ago. She started a blog called Early Mama to document her sense of isolation when she took Noah out and nosy strangers asked incredulously how old she was. She usually said 25 or 27, just to shut them up. “But while I can lie to strangers, the fact remains that I am still a very young mother,” she wrote on her blog. “Most of my friends are in grad school, cheating on their boyfriends, getting wasted. I’ll have to face future play dates where other moms are 10, even 15 years old than I am. But for me, for us, we’re ready and we’re happy, despite your upturned nose.”

The biological “best age” for a baby is clearly out of step, then, with what might be the sociological "best age." Many twentysomethings consider themselves way too scattered and irresponsible to have a child. As Mirowsky put it, "Humans mature reproductively about a decade before Americans mature socially."

You could also define "best" as meaning the best chance for the health of the infant, rather than the health of the pregnancy itself. With that definition, according to Mirowsky, one California study concluded that the “best age” for first birth, in terms of lowest rates of birth defects, is 26. Nice to know, I guess; that’s how old I was when my older daughter was born—and she was pretty perfect. A different study, based on national data, looked at a different measure of a baby’s health—rates of overall infant mortality rather than birth defects—and pinned the “best age” even older, at 32.

Or you could define “best age” as the best outcome for the mother’s long-term health—which puts the ideal age older still. Using data from a phone survey involving women aged 45 to 95, Mirowsky found that those who reported feeling most fit and energetic in middle or old age, and with the fewest self-reported physical ailments, had had their first babies on average at age 29. Those who reported the best health overall in middle or old age had had their first babies on average at 30; those with the fewest aches and pains and the fewest chronic diseases had had them on average at 34. Put it all together, he said, and the optimum age at first birth in terms of the mother’s long-term physical well-being was 31 years old.

And if you define “best age” in terms of the longest life expectancy for the mother, the optimum age is oldest of all. Mirowsky conducted interviews with 1,890 mothers, asking about their current health, including chronic illnesses, mobility problems, and self-assessments of malaise and other problems. Then he looked at mortality data, made some adjustments for educational attainment, and concluded that the overall “best age” for a first child, in terms of long-term health and mortality for the mother, was 34. Social pressure that delays the beginning of parenthood, he wrote, “greatly outweighs the biodevelopmental advantages of youthful organs.” For twentysomethings thinking about how to time their schooling, career advancement, and family building, he offered this advice: they can “reasonably expect optimal health outcomes from delaying motherhood into their thirties.”

Mirowsky put it bluntly in a comment to a reporter for the Daily Mail: “A woman who had her first child at 34 is likely to be, in health terms, 14 years younger than a woman who gave birth at 18.” The reporter then went on to draw the reader’s attention to the gorgeous Sophia Loren, who had her first baby at 34. At the time the article was written (2005), Loren was 70 years old and still received “as much praise for her health and beauty as she did as a young screen siren.”

A different assessment of the long-term health of older mothers presents a slight complication, at least for women who want to have more than one child. While having a first baby at age 34 might be fine, this other study suggests, what’s even more fine is to have a last baby before age 35.

That was the conclusion made by Angelo Alonzo, a sociologist at Ohio State, who in 2002 conducted a study similar to Mirowsky’s using a different data set, the large National Health and Nutrition Examination Survey (N-HANES). After adjusting for social factors that affect health (race, age, income, health insurance coverage, smoking), he compared the health status of two groups of mid- and late-life women: those who had any births after age 35, and those who had finished having babies by then. So this wasn’t looking at age of first birth; it was looking at 35 as a cutoff for age of last birth. Women who had had babies after age 35, Alonzo found, had higher systolic blood pressure, higher blood glucose, poorer health as assessed by a physician, and poorer mobility later in life than women who had had all their babies before 35.

This doesn’t contradict Mirowsky’s findings, exactly. It just means that two different studies came to two slightly different conclusions, and it would be mighty tricky to abide by both of them. For the sake of her long-term health, this collective wisdom goes, a woman should have her first pregnancy at 34—and her last pregnancy before 35.

Adapted from Twentysomething: Why Do Young Adults Seem Stuck? by Robin Marantz Henig and Samantha Henig, Hudson Street Press, Penguin Group (USA) Inc. Copyright © 2012 by Robin Marantz Henig and Samantha Henig.

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