ANN ARBOR—New research on how birth control pills affect the level of hormones in women's blood serum has found much higher levels of hormones in women who take birth control pills compared to women who don't.

The University of Michigan study was motivated by evidence that breast cancer risk increases with hormonal exposure. The authors examined seven commonly prescribed birth control pills and found that four formulations more than quadruple levels of progestin, a synthetic version of the hormone progesterone, and another formulation resulted in a 40 percent higher exposure to ethinyl estradiol, a synthetic version of estrogen.