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Hefty weight gain in pregnancy causes big babies

It’s obvious that when women gain excessive weight during pregnancy there’s a greater chance their babies will be born too big. But scientists have been unsure whether the mother’s weight gain causes high birth-weight or some genetic factors shared by the mother and baby cause both to gain too much weight.

A study published Wednesday comes down on the side of the first theory: It’s a woman’s weight gain that triggers her baby’s size. In the first study of its kind, researchers at Children’s Hospital Boston examined data from 513,501 women who had two or more live births. The number of babies in the study totaled more than 1.1 million. The researchers compared pregnancy weight gain and birth weights among siblings to control for genetic influences.

Clearly, the mother’s weight gain influenced the baby’s weight. Compared with women who gained 18 to 22 pounds (a healthy weight gain for normal-weight women) pregnant women who gained 44 to 49 pounds were 1.7 times more likely to have a high-birth-weight baby. High birth-weight was defined as 8.8 pounds or more. Women who gained more than 53 pounds were 2.3 times as likely to have a high-birth-weight baby.

The issue is of major importance to women and their children. There are well-known health problems linked to excessive pregnancy weight gain for women, such as complications at birth and an increased risk of later diabetes and obesity. For babies, studies are just now beginning to show that the effects of tipping the scales at birth may linger throughout life. Many experts suggest that excessive nutrition in pregnancy creates an abnormal uterine environment that permanently changes the baby’s brain, pancreas, fat tissue and other biological systems, said a co-author of the study, Dr. David Ludwig.

“Hormones and metabolic pathways, and even the structure of tissues and organs that play a role in body weight maintenance are affected,” he said in a news release.

The study was released Wednesday in the journal The Lancet.

Doctors are paying much greater attention to maternal weight gain these days. The Institute of Medicine last year released updated guidelines suggesting stricter adherence to weight-gain recommendations for all women and new guidelines to curb weight gain in obese women. The March of Dimes has detailed information on pregnancy weight gain on its website.

-- Shari Roan

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