MOTHERS who gain weight after having their first baby risk serious complications if they become pregnant for a second time, a study indicates.

Researchers found that even women who were not overweight but gained pounds after their first birth were at risk.

The scientists, writing in The Lancet medical journal, said that the study of 150,000 Swedish women had profound public health implications.

During the course of the study, from 1992 to 2001, the proportion of overweight and obese pregnant women in Sweden increased from 25 to 36 per cent.

Their findings were based on body mass index (BMI) readings of the women taken after their first and before their second pregnancy. The study found that a gain of only one or two BMI units increased the risk of pregnancy-linked diabetes, high blood pressure or high birth-weight babies by an average of 20 to 40 per cent.

An increase of three or more BMI units produced a 63 per cent greater chance of stillbirth compared with a gain of less than one BMI unit.

Eduardo Villamor, from the Harvard School of Public Health in Boston, Massachusetts, who carried out the study with Sven Cnattingius, from the Karolinska Institute in Stockholm, said that previous studies had examined the risk of pregnancy complications by looking at a woman’s weight at a single point before she became pregnant. He said: “In this study, we went further back in time to look at how women arrived at that prepregnant weight. It turns out that women do not need to become overweight or obese to increase their chances of having poor gestational outcomes; only a relatively modest increase in weight between pregnancies could lead to serious illnesses.

“In addition, weight loss in overweight women appeared to lower their risk of pregnancy complications.”

The study used an example of a woman who was 5ft 5in (1.65m) and weighed 10 stones (63kg), giving her a normal BMI of 23.

If she gained 6.6lb between pregnancies — equivalent to one BMI unit — her average risk of diabetes increased by more than 30 per cent. A weight gain of 12.2lb (two BMI units) increased the risk by 100 per cent. The danger would continue to climb if she were to put on more weight.

The researchers said that other factors could be associated with pregnancy outcomes but insisted that the results provided strong evidence that weight gain before pregnancy could lead to complications.

Dr Villamor added: “A key public health message from our study is that women of normal weight should avoid gaining weight between pregnancies. In addition, overweight and obese women are likely to benefit from weight loss if they are planning to become pregnant.”

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