WASHINGTON - New advice for pregnant women: If you're getting a C-section, special inflating boots strapped on your legs may lower the risk of a blood clot.

Hospitals already use these compression devices for other major operations, such as hip replacements, and a growing number have begun offering them for at least some of their cesarean deliveries, too.

Now guidelines for the nation's obstetricians say it's time to make the step routine for most C-sections, which account for nearly a third of U.S. births.

The new recommendations promise to raise awareness of what is a silent threat not just for pregnant women but for thousands of other people, too: Blood clots in veins that can masquerade as simple leg pain.

Called a DVT, for deep vein thrombosis, this kind of clot usually starts in the leg or groin. But it can kill if it moves up to the lungs, where it's called a pulmonary embolism.

These clots make headlines every few years when seemingly healthy people collapse after long airplane flights or similar prolonged inactivity. Certain surgeries also can trigger a DVT. Earlier this year, tennis star Serena Williams was treated for clots in her lungs discovered after foot surgery and cross-country travel.

Obesity, some types of injuries, even some birth control pills can increase the risk, too.

A woman's risk of a DVT jumps during pregnancy and the six weeks afterward. That's partly because of slower blood flow from the weight gain, and because mom is less active in the last trimester and during those first few weeks of recovery from childbirth.

It's also because pregnancy temporarily changes blood to make it clot more easily.

"This is a consequence of nature's protecting women against the bleeding challenges of childbirth," explains Dr. Andra James of Duke University, who co-authored the new guidelines from the American College of Obstetricians and Gynecologists.

Add a C-section and, like any major surgery, it further increases that risk.

As many as two of every 1,000 pregnant women will experience a DVT, James says. Fortunately, pregnancy-related deaths are very rare in this country, but when they happen, those clots are one of the leading reasons.

Yet too few people even know the warning signs, she says: Pain or swelling in one leg, especially the calf or thigh. Redness or warmth in one spot on the leg. If the clot has reached the lung, shortness of breath or chest pain.

The new guidelines urge obstetricians to closely monitor their patients for DVTs - and to check if they have additional factors that would put them at extra risk. Women who have had a DVT earlier in life, or whose close relatives had one - or who have certain inherited clotting disorders - may need anti-clotting medicines throughout the pregnancy, say the recommendations, published in the September issue of the journal Obstetrics & Gynecology.

Then there are those compression devices, which slip over each leg and regularly inflate and deflate, sort of like a massage, to help blood flow more briskly.

The obstetricians' group acknowledges that there haven't been large studies with C-sections to prove how much difference the gadgets could make. But it decided to recommend them anyway because in other types of surgery, the devices can cut the clot risk by two-thirds, James says.

The guidelines recommend strapping them on before the C-section begins, unless there's no time before an emergency operation or the woman is taking anti-clotting medication.

Mount Sinai Medical Center in New York began using the devices for C-sections a few years ago, and deputy chief medical officer Dr. Erin DuPree says they add $14 to the cost of care. Women typically remove and replace them as they ease out of bed throughout the first day after surgery, and no longer need them by the second day, she says.

"It's an easy thing to do that really does not cause harm and could potentially help," she says.

James stresses that leg compression shouldn't lull a woman into thinking she can rest in bed rather than push herself to walk - and a new mom should remain alert to symptoms when she goes home because half of pregnancy-related DVTs occur in those weeks after childbirth.

Lauran Neergaard covers health and medical issues for The Associated Press.