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For the second straight year, the U.S. earned dismal marks in its effort to reduce the national rate of premature births. The March of Dimes, which issues an annual state-by-state report card on the problem, gave the U.S. an overall D on Monday.

That grade was based on figures from 2007, which show that 12.7% of U.S. births were preterm. Those figures, which have remained constant in recent years, also earned the U.S. a D last year, when the March of Dimes began compiling its report card. The objective, set by federal health experts in the Healthy People 2010 program, is a preterm birth rate of 7.6%. Worldwide, the preterm birth rate is estimated at 9.6%, accounting for 12.9 million babies per year. "Preterm birth remains a very intractable problem," says Dr. Jennifer Howse, president of the March of Dimes Foundation. "It does not surprise me that we are not seeing any change [in the U.S.] — yet."

The rate of preterm births, which measures the proportion of babies born before 37 weeks' gestation, is a reflection of a number of factors, both biological and cultural. Starting in 2008, the March of Dimes began tracking three of the major contributors to the high preterm birth rate — lack of insurance among women of childbearing age, rates of cigarette smoking and the rate of babies born preterm, but at the tail end of pregnancy, between 34 and 36 weeks.

Only one state, Vermont, earned a B, while the majority of states earned a C or lower. But the good news, says Howse, is that regarding all three contributing factors, there are encouraging signs that things are moving in the right direction. In 33 states, the percentage of women who smoked dropped from last year; the percentage of insured women of childbearing age with health insurance increased in 21 states; and in a quarter of states, the late-preterm birth rate declined since 2008.

By far the biggest contributor to the high premature birth rate is the rate of so-called late-preterm births. About 70% of babies born too early in the U.S. are born between 34 and 37 weeks. There are many reasons for these early deliveries, making it particularly difficult to target one or even a few factors and address them head-on. The increase in multiples — twins, triplets or more — is one contributor. The rise in assisted reproductive technologies, such as in vitro fertilization, is another; these techniques are associated with both an increased risk of multiples as well as a higher risk of premature delivery, even of singletons. Status of health insurance matters as well. Moms-to-be who are insured have access to proper prenatal care. If a woman sees a doctor regularly, then any problems that arise — pregnancy-related hypertension, for instance, or diabetes — can be picked up early and treated, helping the baby to remain safely in utero for the full 37 weeks.

"The frustration on everybody's part is that no one thing can be fixed to make the rate drop precipitously," says Dr. Laura Riley, medical director of labor and delivery at Massachusetts General Hospital. "Multiple things need to be investigated, and multiple things need to be fixed."

Experts point also to the increasing number of women who elect to induce labor or give birth by cesarean section before 39 weeks. While a baby is technically considered full-term at 37 weeks' gestation, the American College of Obstetricians and Gynecologists advises women not to deliver before 39 weeks. Many women, however, still choose to give birth between 37 and 39 weeks, for nonmedical reasons ranging from convenience to simply wishing not to be pregnant any longer. "But babies that are meant to stay in should just stay in," says Riley. "More maturity goes on between 37 and 39 weeks — the lungs continue to mature, and the brain continues to mature."

Most of these scheduled deliveries occur after 37 weeks and are therefore not considered preterm; however, it is not clear what proportion of late-preterm births, occurring between 34 and 37 weeks, are due to C-sections that are elected for nonmedical reasons.

The more pressing problem, however, is addressing preterm births that occur because of poor prenatal care, says Riley. The rate of these avoidable early births are highest among non-Hispanic African-American women in the U.S. "No matter what year you look at, these women are at the top of the graph," she says. "We haven't made much progress there."

But there may be solutions that physicians could begin implementing today to the problem of preterm births among these populations. A drug known as 17-alpha hydroxyprogesterone, for example, has been shown to reduce the rate of premature birth in women who have previously had high-risk pregnancies or unexplained early births. On the fertility front, transferring fewer embryos for each pregnancy cycle could help lower the multiple-gestation rate and thereby bring down the risk of premature births. Educating women about the importance of a full-term pregnancy and the risk factors that are associated with premature births is also critical; the March of Dimes is sponsoring a study of such patient counseling strategies at hospitals in Kentucky. "These are programs that can be put in place by governors, legislatures, health departments and health care providers," says Howse. "I'm very optimistic about the longer-term result of these efforts on reducing the preterm birth rate."

• Alice Park
• Among the many reasons for the high preterm birth rate in the U.S.: lack of insurance, cigarette smoking and elective cesarean-section deliveries that are scheduled too early