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The U.S. may have the biggest economy in the world, but in one area it is a loser: it’s one of the only countries that do not guarantee paid leave for new mothers. For many people, that’s a powerful symbol of the nation’s failure to support women, especially when compared with Nordic countries where new mothers get up to a year or two to look after their babies.

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But almost all public policies have unintended consequences, and maternity leave is no exception. Rather than offering a route to equality between the sexes, the data shows, extended maternity leave actually throws up roadblocks in a woman’s career — the very roadblocks that such policies are meant to prevent.

Obviously, career trajectory is not the only reason to establish paid maternity leave. It also improves children’s health and well-being. It makes women happier. Every international organization that ranks women’s well-being puts generous maternity leave at the top of its wish list of policies.

We’ve all heard about the seemingly Arcadian maternal leaves in the Nordic countries. In Norway, new mothers can get 47 weeks off. Finland gives less, only 42 paid weeks, but the country has still been ranked No. 1 in Save the Children’s “State of the World’s Mothers” rankings. Sweden gives 480 days at about 80% of a woman’s former salary; the time can be used at any point until a child’s eighth birthday. Denmark may take the prize: it offers mothers a full year at full pay.

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Economists generally agree that these laws have helped bring more mothers back to the office, as they were designed to do. Close to 85% of women ages 25 to 54 were in the labor force in that part of northern Europe as of 2010; Sweden’s 87.5% may be the highest percentage of women in the workforce in the world, compared with 93.6% of Swedish men. In the U.S., by contrast, 75.2% of women are working, compared with 89.3% of men.

But it turns out that extended maternity leave in European countries leads to other problems. Women who take a year off from work with a new baby — not to mention mothers of a second child who take a total of two years — experience what economists call human-capital depreciation, meaning their skills get rusty. Their work-social networks also fray. Unsurprisingly, their income and careers take a hit. “Women who make full use of their maternity or parental leave entitlements receive, on average, lower wages in the years following their resumption of work than those who return before leave expires,” the Organisation for Economic Co-operation and Development concludes in a review of studies on the subject. The effect can continue for years after leave takers return to their jobs and “can permanently damage [mothers’] ability to achieve their labor market potential.”

In fact, generous maternity-leave policies have a tendency to harden a country’s glass ceiling, and women in the Nordic countries are actually less likely to reach career heights than women in the U.S. (The one exception is in the political realm, where quotas have filled Nordic legislatures and ministries with close to equal numbers of women as men.) The U.S. has a higher proportion of female managers at all levels, as well as professionals and university professors, than northern (and the less egalitarian southern) European countries. Though the overall gender wage gap is somewhat higher in the U.S. than in the Nordic countries, that’s not the case among top earners. Female executives and professionals in America earn closer to their male peers than Swedish, Finnish, Norwegian and Danish women. It seems that long maternity leave puts women on a mommy track from which they have a hard time exiting.

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If you’re thinking that one answer to this dilemma is that new fathers should take more of the career hit so new mothers can take less, you’re not alone. Over the past decades, Norway, Sweden and Iceland have been tinkering with policy formulas to get dads to take longer paternity leave. They’ve found that when, and only when, they introduce “use it or lose it” daddy months — that is, when fathers get several months of leave that cannot be transferred to mothers — men will take substantial time off with the baby. However, any leave time left to a couple to divide is almost always taken by women. In Sweden, where the most reliable data is available, women take 76% of parental-leave days. We don’t know yet the effect of an upcoming Icelandic law that would reserve three months for mothers, three for fathers and three to be decided by the couple, though if history is any guide, women will almost always take the extra months.

This doesn’t mean long maternity leave is a bad idea. But it does mean it’s not just women who can’t have it all — it’s policymakers too.