Mama may say it's the right thing to do, but forget good manners and uncross those legs.

Sitting like a proper lady or gentleman interrupts the flow of blood to the heart and may promote varicose veins, according to one expert.

One in four adults has vein problems, although women are 4 to 10 times more likely to develop then, said Luis Navarro, a surgeon who runs the Vein Treatment Center at Beth Israel Medical Center in Manhattan.

Varicose veins are twisted and enlarged veins that are typically close to the surface of the skin but can affect deeper blood vessels as well.

"Crossing the legs is like second nature for most women and some men, too." Navarro said. "It's a bad habit and one of the first things you should learn to undo if you suffer from varicose veins."

Why" Crossing the legs slows the upward flow of blood and increases pressure inside the veins, Navarro explained.

It doesn't matter if you cross at the knee or the ankles, Navarro said. Both are bad, although knee-crossing puts more stress on the venous system.

Melissa Kapalan, 23, who sat reading, cross-legged, recently at the Rockefeller Center in Manhattan, said she had "no idea there was any connection" between the way she sat and possible health risk.

Navarro said half of women over 40 suffer from varicose veins or spider veins, which are less serious breaks in smaller, surface veins.

The cause is unknown, but genetics is a key factor. "Sixty to 80 percent of people who have varicose or spider veins have a history on their mother's side," Navarro said.

"Exercise is the first and most important thing to do," Navarro said. People who sit eight hours a day with their legs crossed will have a greater chance of developing varicose veins if there is a genetic factor, he said.