Women who start their families early and those who have lots of kids have an increased risk of developing the most common type of kidney cancer, renal cell carcinoma, new research suggests.

Renal cell carcinoma occurs in men twice as often as in women. Known risk factors for the disease include obesity, high blood pressure, and smoking, but it’s not clear whether sex hormones might also play a role.

To investigate, Dr. Jung Eun Lee and colleagues from Harvard Medical School in Boston note looked at 118,219 women participating in the Nurses’ Health Study, 247 of whom were diagnosed with renal cell cancer between 1976 and 2004.

The researchers found that women who had given birth to more children were at greater risk of renal cell carcinoma, with the likelihood of the disease climbing by 10 percent for every child born.

They also found that the later in life a woman had her first child, the less likely she was to be diagnosed with the disease; delaying childbirth to age 28 or later cut risk by 34 percent compared to women who had their first child when they were 22 or younger.

However, there was no relationship between use of hormones after menopause or birth control pill use and kidney cancer risk, according to a report in the American Journal of Epidemiology.

It’s possible, the researchers say, that pregnancy could boost renal cell carcinoma risk by putting more stress on the kidneys; urinary tract infection also is more common during pregnancy, they point out. The kidneys get slightly bigger during pregnancy, they note, which could make the organs more vulnerable to oxidative stress and inflammation.

The extended changes in estrogen and progesterone levels that would occur with multiple pregnancies could also influence disease risk, the researchers add, even though using hormone therapy or oral contraceptives wasn’t related to renal cell carcinoma in the current study.

SOURCE: American Journal of Epidemiology, May 15, 2009.