Using data from more than 24,000 UK women who gave birth between 1976 and 2005, researchers found that the risk of problems, such as high blood pressure, pre-eclampsia and premature delivery climbed in tandem with a woman's pre-pregnancy weight.

The findings, published in the online journal BMC Public Health, add to evidence that obesity is a risk for mothers and newborns.

They also support the belief that all pregnancies in obese women should be considered "high risk," and managed accordingly, conclude the study authors, led by Dr. Sohinee Bhattacharya of Aberdeen Maternity Hospital.

The researchers found that compared with normal-weight women, obese women were 50 percent more likely to have post-delivery bleeding and twice as likely to deliver prematurely. They were also more likely to need an emergency C-section or to have labor induced.

Morbidly obese women had the highest risk of suffering pre-eclampsia, a potentially serious pregnancy complication marked by a sudden rise in blood pressure and kidney abnormalities.

In contrast, the study found that women who were underweight before pregnancy tended to have the lowest risk of all these complications. They were, however, more likely than normal-weight women to have an underweight newborn.

The results add to growing evidence of the importance of a mother's weight in pregnancy outcomes, according to Bhattacharya's team.

"The evidence for obesity as an important complication in pregnancy is mounting," the researchers write, it is time for physicians to be aware of these findings and start using them in their practice.

Besides good prenatal care, they note, this means counseling overweight women to lose weight before they become pregnant.