WASHINGTON  —  One in eight babies — well over half a million a year — are born premature, a toll that's risen steadily for two decades with no sign of stopping.

The government this week begins an unprecedented push to figure out why, with special aim at preterm births that may be lowered: so-called late preemies, those born weeks, not months, early.

"The average woman should be thinking about this," says acting Surgeon General Steven Galson, who opened a two-day conference Monday on developing a national strategy to prevent premature birth. "We really need to redouble our efforts."

There are some steps mothers-to-be can take to reduce their risk, if only more got the message.

A full-term pregnancy lasts from 38 to 42 weeks. Babies born before completion of week 37 are premature. Scientists know far too little about what triggers early labor or how to stop it, especially for very early preemies — those born before 32 weeks who, despite advances in the neonatal ICU, are most likely to die or suffer devastating disabilities such as cerebral palsy or retardation.

On the other hand, there are troubling signs that a portion of the increase in late preemies — those born between 34 and 37 weeks — may be due to unnecessary Caesarean sections.

If slightly early birth sounds like no big deal, consider: A baby's brain at 35 weeks weighs only two-thirds of what it will weigh at 40 weeks, says a dramatic "brain card" developed by the March of Dimes to educate women. Some will catch up fine, while others have learning or behavioral delays. Also, being even a few weeks early can cause initial breathing problems if lungs aren't mature enough; feeding problems; even an increased risk of SIDS.

"Most women are not really aware of how two to three weeks can make a big difference in the full development of the baby," says March of Dimes President Dr. Jennifer Howse.

These near-term babies make up 70 percent of all premature births and they, not very early preemies, are fueling the two-decade rise, says a just-published study from the federal Centers for Disease Control and Prevention and the March of Dimes.

"What's driving that is where we're going to get our biggest bang for the buck," says Dr. Catherine Spong, pregnancy chief at the National Institute of Child Health and Human Development.

C-sections soared in that time frame, too, and the new study links the two. Researchers examined nine years of singleton births — less complicated than twins or more — and found that C-sections accounted for almost all the increase in late preemies.

The question then becomes who truly needed an early C-section for medical reasons, something the study couldn't tally. There are clear guidelines that a C-section shouldn't be scheduled before full-term unless there's a medical need. But uncomfortable moms-to-be sometimes request delivery before their due date. Doctors worried about litigation sometimes agree for minor reasons, like a slight uptick in the mother's blood pressure.

There also can be confusion about the fetus' exact age — prompting the prestigious Institute of Medicine in 2006 to urge that more pregnant women receive a first-trimester ultrasound exam so that wouldn't be in question months later when delivery decisions are made. Whether and how to enact that advice is one key question for the surgeon general's meeting.

Another reason for the increased federal attention to preemies: Babies born too small grow up to be at increased risk of heart disease, diabetes, strokes.

"The public health importance of preventing preterm birth can't be undersold," says Spong.

Certain to come from the meeting is some sort of consumer education about the risk factors doctors do know, and steps that can help.

Smoking and lack of prenatal care are the top preventable risks. To push health authorities to address those, the March of Dimes this fall will begin publicly grading states on their rates of preterm birth, pregnant women's access to smoking cessation programs, how many are uninsured and how many get a first-trimester ultrasound.

Other factors:

— Folic acid supplements already are advised to prevent birth defects, but new research shows consuming folic acid for a year before getting pregnant dramatically lowers risk of premature birth. That's longer than most women are believed to take the nutrient, and thus an opportunity.

— Carrying twins or more increases prematurity risk, a risk worth discussing with women undergoing in vitro fertilization where more than one embryo typically is implanted.

— Pregnancy before age 16 and after 35 increases prematurity risk, another opportunity to discuss the importance of pregnancy timing.