A woman's weight during her first pregnancy may affect the health of a second baby, even if everything went well the first time around, a new study has found.

It is already well established that a mother's weight can increase the risk of complications during pregnancy. For example, if a woman is overweight or obese, this increases her risk of developing gestational diabetes.

According to US researchers, many women are not a healthy weight when they first become pregnant. They may be overweight or underweight, although more often, they are overweight.

The study found that complications may occur during a second pregnancy even if none occurred during the first pregnancy, and even if the mother is a normal weight the second time around.

"While moms who weigh too much or too little might have uncomplicated pregnancies and deliver healthy babies the first time around, our research shows they are still at increased risk of adverse outcomes during their second pregnancies. That risk may not go away even if they are at a normal weight when they get pregnant again," commented the study's senior author, Dr Jen Jen Chang, of Saint Louis University.

She said that it is unclear why a woman with an unhealthy weight who has had no complications during her first pregnancy, should develop them during a second one.

"I suspect our body remembers, but we don't know for certain. Women who are over or underweight during their first pregnancy may experience permanent physiological changes that negatively affect their second baby," she suggested.

The study involved over 121,000 women who had given birth between 1989 and 2005. It found that those who were underweight during their first pregnancy had a 20% increased risk of giving birth prematurely and a 40% increased risk of having a smaller than average baby the second time around, when compared with women of a healthy weight.

Meanwhile, those who were obese during their first pregnancy were much more likely to face complications during a second pregnancy, compared to women of a healthy weight. For example, they were 54% more likely to have a larger than average baby and they were 156% more likely to have pre-eclampsia the second time around.

Their babies were also 37% more likely to die within the first month of life.

"Our finding that obese women are at increased risk of neonatal death in a subsequent pregnancy, even if their first pregnancy was uncomplicated, has significant public health repercussions and warrants further research," Dr Chang noted.

The researchers said that ideally women should try to maintain a healthy weight before becoming pregnant for the first time. However those who are found to be overweight or underweight should be closely monitored during subsequent pregnancies.

"The bottom line for physicians and second-time moms is not to let down their guard even if things went well for moms with unhealthy weight during the first pregnancy or if those moms reach a normal weight when they become pregnant again," Dr Chang added.

Details of these findings are published in the American Journal of Obstetrics and Gynecology.

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