IStock Photo 4444449 © Courteny Humphries

Premature births are becoming more common around the globe, a phenomenon that puzzles health experts. In 1990, 1 in 9.43 births in the US occurred prematurely, but by 2007, 1 in 7.87 births was premature. The March of Dimes estimates that rates of prematurity have risen 35 percent since 1981.

Even the norm of pregnancy duration has decreased. A typical pregnancy now lasts about 39 weeks, as opposed to the previous norm of 40, a gradual change that is estimated to have occurred between 1992 and 2002. The full length of pregnancy is important to give the baby time to develop and mature in the protected environment of the womb; any baby born after less than 37 weeks of gestation is considered premature or preterm, while babies born less than 34 weeks into the pregnancy are considered early preterm.

The increase in early births has happened alongside dramatic medical advances making it possible to keep premature babies alive and healthy; many babies survive and thrive even after a shortened stint in the womb. However, even with the best medical care, preemies still face a higher risk of infections, life-threatening health problems, and lifelong disabilities. Caring for premature babies also carries a steep cost to society, more than $26 billion in this country in 2005 alone.

Better medical care after birth doesn't explain why premature births take place more often in the first place. Prematurity rates are climbing worldwide, even in less developed nations that lack sophisticated medical technologies to care for these infants.

Scientists have identified several reasons why women give birth early, but there is no one factor that explains the dramatic worldwide increase. Research has linked prematurity to diabetes and obesity in women in richer countries, nutrient deficiencies in poorer ones, and chemicals in the environment. Health factors such as cigarette smoking, chronic stress, and certain infections can put a woman at higher risk of giving birth prematurely. Twins, triplets, and other multiples have a far higher risk of early birth, and women having babies later in life are also more likely to give birth prematurely. But even among singleton babies, prematurity is more common now than in the past. Overall, about 40 percent of premature births happen for unknown reasons, and research is ongoing to uncover the causes.

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