Women who put on even a moderate amount of weight after having their first baby increase the risk of complications - such as stillbirth or needing a caesarean - in having a second child, scientists warn today.

A study of 150,000 women in Sweden has found that even those who are not medically overweight but put on the pounds between babies run increased risks of several harmful conditions. These include pre-eclampsia, temporarily high blood pressure and diabetes during pregnancy, caesarean delivery, stillbirth and babies that are large for their gestational age.

The authors, Eduardo Villamor from the Harvard School of Public Health in Boston and Sven Cnattingius from the Karolinska Institute in Stockholm, say the public health implications of their work are profound.

"Women do not necessarily need to become overweight or obese to increase their risk of gestational complications," they write in the Lancet. "Instead, a modest increase in weight between pregnancies within the healthy BMI [body mass index] category, or enough to shift from the healthy to the overweight category, would be sufficient to increase a woman's average risk of serious adverse outcomes during the next pregnancy.

"Our study underscores the importance of avoiding weight gain between pregnancies and accords with the view that even a moderately increased BMI could be deleterious for maternal and neonatal health."

During the course of the study, from 1992 to 2001, the proportion of overweight and obese women in Sweden rose from 25% to 36%.