New mothers who worry about their weight is not breaking news. But a recent study, just published in the January issue of The American Journal of Preventive Medicine, shows that childbirth or the legal state of marriage has little to do with packing on the pounds for most women. It seems that even childless women who live with a mate put on more pounds than those who live without one.

Apparently, the differences in weight gain between women who lived alone and women who cohabitate with men are drastic. The average-sized woman is 140 pounds. According to the study, after adjusting for various variables, the 10-year weight gain for an average 140-pound woman was 20 pounds if she had a baby and a partner, 15 if she had a partner but no baby, and only 11 pounds if she was childless with no partner.

This study included more than 6,000 Australian women over a 10-year period ending in 2006. Starting out, the women ranged in age from 18 to 23. Each woman periodically completed a survey with more than 300 questions about weight and height, age, level of education, physical activity, smoking status, alcohol consumption, medications used and a wide range of other health and health care issues.

Annette J. Dobson is a professor of biostatistics at the University of Queensland in Australia and the lead author of the study. Dobson told the New York Times that there is no reason to believe metabolic changes are in sync with the state of cohabitation — rather, physiological changes might be at work.

Some suggest a more active social life may help explain why women with partners gain more weight. Maureen A. Murtaugh is an associate professor of epidemiology at the University of Utah who is an expert on weight gain in women. “Think of going to a restaurant,” Murtaugh told the NY Times. “They serve a 6-foot man the same amount as they serve me, even though I’m 5 feet 5 inches and 60 pounds lighter.”

Murtaugh does not take into account that most of the time, a woman is not eating the exact same thing as her partner.

There were other interesting findings among the women at the end of the study. There were fewer smokers and risky drinkers than at the beginning, more women who exercised less and a larger number without paid employment. But despite these other factors, the differences remained in weight gain among women with and without babies, and among women with and without partners.

Experts find the results of this study troubling. As Dobson concludes, “From a prevention point of view, one can look at these as particular times when women need to be especially careful.”

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