By Amy Norton

NEW YORK (Reuters Health) - Women who go through a traumatic event during or soon before pregnancy may be at increased risk of having an underweight baby, a large study suggests.

Researchers found that of more than 1 million Danish women who gave birth over 24 years, those who dealt with the death or serious illness of a loved one shortly before or during pregnancy were more likely to have a low-birthweight baby.

The findings, reported in the journal Psychosomatic Medicine, do not prove that severe stress during pregnancy harms fetal growth. However, it is possible that this is the case, according to lead researcher Ali S. Khashan, of the University of Manchester in the UK.

Research indicates that high levels of stress hormones in the mother can hinder fetal growth, and severe stress may make it difficult for some pregnant women to follow a healthy lifestyle.

Still, Khashan told Reuters Health, the current findings are "subtle," linking severe stress to a relatively small effect on birthweight. "So the overall risk to an individual experiencing 'normal' stress is tiny," the researcher explained.

On the other hand, Khashan said, the study does give "valuable insights" into the importance of the uterine environment in fetal development and, ultimately, babies' well-being.

The researchers based their findings on records for 1.38 million women who gave birth in Denmark between 1979 and 2002. Denmark's system of national registers allowed the researchers to link each woman with data on her first-degree relatives -- parents, siblings, spouses and other children -- and identify those who'd faced a family member's death or serious illness during pregnancy or within the six months before pregnancy.

Overall, the study found, women who'd lost a family member were 22 percent more likely than other women to have a baby who weighed below the 5th percentile at birth. Serious illness in a family member was also linked to poorer fetal growth.

While the findings do not prove a cause-and-effect relationship, they do underscore the importance of trying to stay healthy during pregnancy, according to Khashan.

"Expectant mothers should follow standard medical advice and maintain a normal, healthy lifestyle pre-conception and in pregnancy," the researcher advised.

SOURCE: Psychosomatic Medicine, July/August 2008.

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