London ? Women who gain as little as 7 pounds between pregnancies can put themselves and their babies at medical risk, even if they don’t become overweight, suggests a provocative study of thousands of women.

Researchers found that gaining weight during that interval – not during the pregnancy itself – raised the risk of such complications as diabetes, high blood pressure and even stillbirth during the second pregnancy.

Pregnant women with diabetes or high blood pressure are at risk of convulsions or organ damage, which in severe cases can be fatal.

The results provide new evidence that overweight or obese women who plan to become pregnant should lose weight, and that women with healthy weights should avoid packing on pounds before pregnancy, the researchers said.

The work was reported Thursday in the British medical journal The Lancet by Drs. Eduardo Villamor of the Harvard School of Public Health and Sven Cnattingius of the Karolinska Institute in Sweden. They examined records of more than 150,000 Swedish women who delivered two children between 1992 and 2001.

The researchers focused on body-mass index, or BMI, a calculation from a person’s height and weight. They examined the difference between the women’s BMI at the beginning of two consecutive pregnancies. And they examined the likelihood of complications such as high blood pressure, diabetes, the probability of a Caesarean delivery and stillbirth in the second pregnancy.

One striking finding was that the risk of complications rose even in women who did not end up overweight, Villamor said Thursday.