Philadelphia ? New moms should get up and start walking as soon as possible to prevent the risk of a potentially fatal blood clot, doctors advise.

Although the chances of such clots are rare, they are four times greater for pregnant women and new mothers, a large 30-year study found, confirming what doctors have long observed.

Mayo Clinic researchers looked at medical records from 1966 to 1995 of 50,000 pregnant women who lived in Olmsted County, Minn., where data has long been gathered for a long-term health surveillance project.

The researchers focused on blood clots in leg veins (known as deep vein thrombosis) and clots that broke loose and lodged in the lungs (known as pulmonary embolism).

The incidences of deep vein thrombosis and pulmonary embolism were small – only 105 cases occurred over the 30-year period – but the problem is of concern because it is frequently fatal when it does happen.

In roughly one-fourth of pulmonary embolism cases in general, the first and only symptom is sudden death, said Dr. John A. Heit, lead author of the study appearing in today’s Annals of Internal Medicine.