How long do working mothers stay home after having their first child? If you guessed the answer might be 12 weeks (not an unreasonable assumption, since that’s the amount of time allotted by our national family leave law), you’d be sadly mistaken. According to recently released census numbers, a majority of mothers who worked during pregnancy go back before that, some way before. More than a quarter are at work within two months of giving birth and one in 10—more than half a million women each year—go back to their jobs in four weeks or less.

Let’s take a moment to think about what’s going on just four weeks after birth. Babies haven’t even cracked their first real smiles yet. Mothers are still physically recovering from birth, particularly if they’ve had C-sections. They’re both probably getting up several times during the night to nurse. In fact, they’ve barely begun what’s supposed to be half a year of exclusive breast-feeding, according to the American Academy of Pediatrics.

Yet going back to work in such a short amount of time isn’t just tiring or unpleasant, new research demonstrates that it’s bad for both women and children. We now have enough evidence to blame the short amounts of time mothers have with newborns for developmental delays, sickness, and even death. (I say mothers because, while most leave laws apply to men and women, women are far more likely than men to take time off and, thus, are the subjects of most research.)

So leaving aside for a moment the backward politics in the United States that leave us without any paid time off, what does this growing body of knowledge tell us about how much time would actually be optimal? Some of the results are surprising. For one thing, there is some evidence that very long leaves have an economic and professional downside for women, and at best a neutral effect on children. So it’s not simply that more time off is better. Rather, certain amounts of leave may give the biggest bang, while longer periods of leave may yield diminishing returns, at best.

By looking to Europe, which has meticulous data collection practices and a history of paid leave stretching back to the 19th century, researchers have been getting a better and better handle on the extent to which varying amounts of paid leave can save kids’ lives. Two studies, one published in the Economic Journal in 2005 and another five years earlier , examined the results of the steady climb in paid leave in 16 European countries, starting in 1969. By charting death rates against those historical changes, while controlling for health care spending, health insurance, and wealth, the authors were able to attribute a 20 percent dip in infant deaths to a 10-week extension in paid leave. The biggest drop was in deaths of babies between 2 and 12 months, but deaths between 1 and 5 years also went down as paid leave went up. So what was the optimal amount of time off, according to all this research? According to Christopher Ruhm, the author of the first European study, paid leave of about 40 weeks saved the most lives. (After that point, according to Ruhm, “there may even be some partial reversal of those gains.”)

Here in the United States, the few paid leave programs we have may be too small to make much of a difference, as the authors of a study published this month suggested after being unable to find any impact of state leave policies on children’s health. Efforts to study paid leave in this country are further complicated by the fact that those American parents who do get paid time off often tend to be lucky in other ways, too. That recent census report shows that only 18 percent of mothers with less than a high school education got paid time off compared with 66 percent of women with at least a bachelor’s degree. This makes it hard to know whether differences between American families in which a parent was able to stay home and families in which the mother went right back to work might instead be attributable to poverty, education, or other factors.

Turning our eyes back to Europe, there is evidence that leave—even when it’s shorter than that apparently ideal 40-week span identified by Ruhm—has not just health effects but measurable developmental and behavioral benefits, too. One study tracked Norwegian children who were born after 1977, when that country increased its paid leave from zero to four months and its unpaid leave from three to 12 months, and found that the kids born after the change had lower high school dropout rates. Military draft data, moreover, tied lengthened leaves to increases in male IQ (and height, too).

It’s not entirely clear why having parents around would help babies grow taller or smarter, or live long longer, but the research points to a few potential advantages to kids whose mothers stay home for at least three months. In another study published in the Economic Journal in 2005, American babies whose mothers were back at work within 12 weeks were less likely to get doctors’ visits and immunizations and be breast-fed. All this makes intuitive sense, of course: Checkups can help diagnose and treat illnesses, but they are hard to schedule when you’re working. And while exclusive breast-feeding for at least six months has been shown to prevent respiratory infections, bacterial meningitis, and other illnesses, going back to work can make it difficult if not impossible.

In the developmental realm, the benefits of leave may be trickier to explain. That 2005 Economic Journal study of American women who returned to work within 12 weeks showed that infants whose mothers went back even earlier were likely to have more behavioral problems and lower cognitive test scores at age 4. The authors speculated that the difference might have stemmed from the superior care babies receive from parents, as opposed to other caregivers. It might also have something to do with attunement, the crucial developmental process through which parent and newborn adjust to each other.

But what about those parents—most of whom are mothers? What do we know about what the ideal length of leave time might be for them? In terms of American mothers’ mental health, the best answer for now may be simply: more. Numerous studies have tied the lack of time off to depression in working mothers. Conversely, a 2004 study found that an increase of just one week of time off decreased the number and frequency of symptoms of depression in American mothers.

It’s easy to understand why an American woman going back to work just four, eight, or even 12 weeks after birth might get depressed—especially if she looks to Europe, where at least six months of paid leave is the norm and several countries grant more than three years.

Maybe we American women can cheer ourselves with the several recent studies that have failed to find benefits of such very long leaves. It turns out that the increase from 12 to 15 months of paid leave—which Sweden made back in 1988—doesn’t have a dramatic effect on kids. There is even some evidence that laws granting more than a year and a half off paid can hinder women’s professional achievement. It may be cold comfort, but at least this is one problem that we American mothers, facing the prospect of caring for new babies while somehow holding onto our jobs, just don’t have.