Eyes squinched shut, Luke Chase yawns and stretches a little short of his full 15 inches. A plush pony nestles against his back in an incubator at St. Joseph's Hospital and Medical Center in Phoenix.

Younger-by-minutes brother Cole snoozes on his stomach in another incubator a few feet away, with parents Debbie and Rick Chase watching his back rise almost imperceptibly with each breath.

"Hey, buddy," Debbie says and strokes Cole's back. "You've got to take some breaths."

When the twins were born March 6, about 12 weeks before their due date, they became part of a troubling upward trend in premature births.

Medical experts know what makes women more likely to have healthy, full-term pregnancies, but they aren't sure why an increasing percentage of pregnancies result in premature births, sometimes in spite of the best efforts of mothers-to-be and their doctors.

A GROWING PHENOMENON

Nationwide, 12.5 percent of babies born in 2004, or about a half million, were premature, up from 9.4 percent in 1981, according to a National Center for Health Statistics report released in October 2006.

Preterm birth is defined as occurring before the mother has completed 37 weeks of pregnancy, and in almost half of the cases, doctors don't know why they happen. Thirty-seven to 42 weeks is full-term.

Jennifer Gray, newly married to Dan Arends, had her twins, Ian and Julia, two and one-half months early. They spent two months in the hospital with myriad health problems.

"I will never sweat the small stuff with my kids," she says. "Watching them fight for their lives really puts things in perspective. We were lucky."

Premature births remain a mystery mostly because "a cascade of factors appears to be at work," says Physician Robert Gutierrez, medical director of St. Joseph's neonatal intensive care unit.

"There are so many variables," he says. "One little thing may lead to another. And every socioeconomic class is affected, not just those without good access to health care."

Gutierrez and other health care experts say infections — of the mother's genital and urinary tracts, fetal membranes and possibly gums — play a role.

Researchers also know that the women most at risk have had a previous premature birth, have certain uterine or cervical abnormalities or are carrying two or more babies.

In fact, moms like Debbie Chase were found in one March of Dimes analysis to be six times more likely to have preterm deliveries than women pregnant with one child. The nonprofit organization works to improve prematurity rates and other issues affecting babies' health.

Debbie was flown to St. Joseph's from Prescott Valley, Ariz., where the family lives, and scheduled for a Caesarean section after being diagnosed with life-threatening HELLP syndrome. The rare condition's name is a reference to its symptoms: hemolysis (breakdown of red blood cells), elevated liver enzymes and low platelet count. Debbie's pregnancy caused it, and giving birth was its only cure.

Luke was delivered first, at 3 pounds, 3 ounces. Next came Cole — kicking and screaming, lungs obviously healthy, Rick says — at 2 pounds, 13 ounces.

The boys will stay at the hospital until they reach milestones, such as breathing on their own, sucking and swallowing efficiently, and gaining weight steadily.

"They're so light, and they seem so fragile," Rick says. "But the doctors and nurses tell us they're pretty tough."

SCARY PROSPECTS

Debbie, a nurse, and Rick, a firefighter, know too well how things might have turned out.

Prematurity is the leading cause of newborn death; one-fourth of babies who die within a month of birth are premature. Premature babies who survive face possible short-term and lifelong consequences, from breathing problems to cerebral palsy.

" 'Survival' is a word you use with caution," Gutierrez says. "All that means is the baby leaves the hospital breathing and eating. It doesn't tell you about any complications they have that may affect their quality of life."

The odds of not just surviving but thriving have improved in recent years, though, thanks to the use of antibiotics, surfactants that help the babies' lungs develop and high-tech ventilators, he says. Better understanding of preemies' nutritional needs and advances in delaying women's labor also have had an impact.

"When I was a resident ... in Los Angeles in the late '80s," Gutierrez says, "a baby who was born at 28 weeks and survived was like a miracle. Now those babies can do really well."