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It wasn’t that the procedures or technologies involved in childbirth became that much more expensive over time. The reason for the increase, according to the study authors, is the rise in high deductibles—the lump sums that insurance companies make their customers pay before the companies will kick in any money. Indeed, more Americans have found themselves on plans with high deductibles in recent years as employers have sought to shift health-care costs onto employees. In the new study, Moniz and her colleagues found that the percentage of women with deductibles rose from about 69 percent to about 87 percent in the seven-year time period. Women paid a greater share—about 7 percent more—of their childbirth expenses as a result.

In the U.S., 28 percent of insured workers are now on plans that have a deductible of at least $2,000, says Usha Ranji, an associate director for women’s health policy at the Kaiser Family Foundation. “Spending on maternity care really tracked with the trends that we’ve seen in private insurance overall,” she told me.

David Anderson, a research associate with the Duke-Robert J. Margolis Center for Health Policy who was not involved in the study, says while this study reinforces the effect of high deductibles on American patients, it has some drawbacks. By including all medical care in the 12 months leading up to delivery, he says, the Health Affairs authors risked overestimating the childbirth-related medical expenses of the women in the study. For example, a broken leg that a woman suffered 11 months before she went into labor would presumably have been included in the study. (Moniz acknowledged this limitation but argues an approach that included only expenses directly related to pregnancy would have undercounted the true cost, because some doctors’ visits in the months leading up to childbirth would not be coded by insurers as pregnancy-related.)

The cost of having a baby can be especially steep for the 45 percent of women whose pregnancies are unplanned. Because they might not have been expecting a baby when they signed up for their health plans, they might not have set aside the money to pay for their delivery or signed up for coverage that would have taken care of more of their delivery costs. (Childbirth is the No. 1 reason for hospitalization among American women.) What’s more, the cost of the delivery is just the first in a series of major child-bearing expenses to come. Not long after these mothers have paid their hospital bills, they’ll be shelling out for daycare, sitters, clothes, and school fees. “This is the kind of money that causes people to go into debt,” Moniz says.

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This study, like many others, highlights the limits of American health insurance, including for those who are insured. Even though the Affordable Care Act brought order to the wild west of health insurance, customers can still get stuck with large bills. Some hospitals allow their doctors to bill their patients as out-of-network providers, for example, and even a standard 20 percent co-pay on an expensive medication or treatment can work out to hundreds of dollars.