NEW YORK - A range of health and lifestyle factors can predispose middle-aged women to bladder-control problems, though the causes tend to shift with age, according to a new study.

Urinary incontinence is a common problem, particularly among women. But compared with older women, little has been known about the risk factors that contribute to incontinence in middle-aged women.

The new study included 3,300 U.S. women with an average age of 46 years. The researchers found that past pregnancies, diabetes, poor overall health and being overweight all seemed to raise the risk of incontinence.

However, each factor became more or less important over time, researchers report in the American Journal of Epidemiology. Past pregnancies, for example, were a risk factor for bladder-control problems at the start of the study, but not for cases that developed over the next five years.

On the other hand, excess weight was not a risk factor at the study's outset, but became more important over time, according to the study authors, led by Dr. L. Elaine Waetjen of the University of California, Davis.

The study
The women, between the ages of 42 and 52 years old, were participants in an earlier study in which they were followed for five years and questioned each year about their health, lifestyle and quality of life.

At the beginning of the study, nearly half of the women said they had urinary incontinence at least once a month. The increased risk at this time included having children, diabetes or benign growths in the uterus called uterine fibroids. Women who reported little support from family and friends - a source of psychological stress - were also at risk.

However, the risk factors were different for women who developed incontinence later in the study period, Waetjen and her colleagues report in the American Journal of Epidemiology.

For them, excess weight increased the risk of bladder-control problems, as did poor overall health and a tendency to be sensitive to bodily sensations, like cold and heat.

Throughout the study period, the women most often reported "stress" incontinence, in which urine leaks when the bladder is under pressure - from exercise, coughing or laughing, for example.

However, "urge" incontinence grew more common as the women aged; with this form of the disorder, people feel an overwhelming need to urinate, often before they can reach a bathroom.

African American women were particularly likely to develop urge continence during the five-year study period. Although African Americans had one of the lowest rates of incontinence at the outset, their rate of new cases during the follow-up was one of the highest.

This may, according to Waetjen's team, have to do with the higher rate of uterine fibroids among black women or their generally greater body weight.

The researchers conclude that risk factors for incontinence appear to change as women move through middle-age. That knowledge, they note, may help in treating the condition.