By NICHOLAS BAKALAR

It is widely known that women tend to gain weight after giving birth, but now a large study has found evidence that even among childless women, those who live with a mate put on more pounds than those who live without one.

After adjusting for other variables, the 10-year weight gain for an average 64-kilogram woman was 9 kilograms if she had a baby and a partner, 7 if she had a partner but no baby, and only 5 kilograms if she was childless with no partner. The number of women with a baby but no partner was too small to draw statistically significant conclusions.

The weight gain among childless women with partners was almost surely caused by altered behavior. There was a steady weight gain among all women over the 10 years of the study.

This does not explain the still larger weight gain in women who became pregnant. The lead author, Annette J. Dobson, a professor of biostatistics at the University of Queensland in Australia, suggested that physiological changes might be at work.

“Women’s bodies may adjust to the increased weight associated with having a baby,” Dr. Dobson said. “There may be a metabolic adjustment that goes on when women are pregnant that is hard to reverse .”

The study covered more than 6,000 Australian women over a 10-year period ending in 2006.

At the start, 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.

By the end of the study, published in the January issue of The American Journal of Preventive Medicine, more than half the women had college degrees, about threequarters had partners and half had had at least one baby. Almost all of the weight gain happened with the first baby; subsequent births had little effect.

Also by the end of the study period, there were fewer smokers and risky drinkers than at the beginning, more women who exercised less and a larger proportion without paid employment. But even after adjusting for all of these factors and more, the differences in weight gain remained.

“It’s interesting and brings out some important points,” said Maureen A. Murtaugh, an associate professor of epidemiology at the University of Utah . Perhaps, she suggested, a more active social life may help explain why women with partners gain more weight.

In a study, even women who remained childless gained weight after they found a partner. / CARMEL ZUCKER FOR THE NEW YORK TIMES