Previous research has identified a risk in men, but has not pinpointed the same level of risk in women.
The research team based their findings on almost 90,000 women between the ages of 40 and 59, taking part in the Canadian National Breast Screening Study.
Over the 10 years of monitoring, 527 women were diagnosed with bowel cancer.
When these figures were set against weight, the researchers found that women who were clinically obese, defined as a body mass index of 30 kg/m2 or more, were twice as likely to develop the disease. However this was only true before the menopause.
| Nearly 90,000 women included in the study.
Being very overweight after the menopause did not increase bowel cancer risk, and if anything, slightly decreased it.
The authors suggest that excess body fat is associated with increased levels of blood insulin and related chemicals, insulin-like growth factors, which research has linked to increased bowel cancer risk.
However, after the menopause, fat tissue is an important source of estrogen, which may be protective and counteract the harmful effects of insulin.
By contrast, fat tissue is a negligible source of total circulating estrogen before the menopause, they say.
Therefore, being very overweight in youth and young adulthood may increase a woman's risk for this common form of cancer.
They conclude that previous research indicating little or no increased risk of bowel cancer among obese women might have failed to calculate the impact of menopausal status.