A new study suggests birth control pills could have more dangerous side effects than previously thought.

A research team at University of Michigan set out to analyze how much the oral contraception affects hormone levels in women, since certain hormones increase breast cancer risk.

Analyzing seven commonly-prescribed pills, they found four of them more than quadrupled the levels of progestin - a synthetic form of progesterone - in women.

One pill drove up exposure to ethinyl estradiol, a synthetic estrogen strongly linked to breast cancer, by 40 percent.

A study warns some brands QUADRUPLE levels of synthetic hormones linked to breast cancer

Human evolutionary biologist Beverly Strassmann, who led the study, said the research should not discourage women from using birth control pills, since they are an effective form of contraception and can have other medical benefits.

Rather, she said, the findings should be motivation to overhaul how birth control pills are designed to minimize the risk of breast cancer.

'Not enough has changed over the generations of these drugs, and given how many people take hormonal birth control worldwide—millions—the pharmaceutical industry shouldn't rest on its laurels,' said Professor Strassmann, faculty associate at the University of Michigan Institute for Social Research.

Progesterone and estrogen are both produced by the ovaries and their levels vary naturally over the course of the menstrual cycle.

The pill replaces these naturally released hormones with synthetic versions.

TYPES OF BIRTH CONTROL PILL 1. Combination pills Combination pills contain two types of hormones: estrogen and progestin. They prevent pregnancy in three ways: Different pills will have different amounts of the two hormones over the course of the month. 2. Mini-pills Mini-pills contain only progestin. They prevent pregnancy by thickening your cervical mucus and thinning the lining of the uterus. The mini-pill was developed for women who are sensitive to estrogen. It has the same failure rate as the combination pill - about one in 10. Advertisement

The study's goal was to test whether the synthetic versions increased or decreased hormonal exposure compared to what women might get from their own ovaries.

'That this hasn't been answered is amazing, given that we already know that there's a correlation between hormonal exposure and breast cancer risk,' Professor Strassmann said.

To conduct the study, the research team analyzed data from 12 previous studies that measured the amount of estrogen and progesterone over the menstrual cycle in women who were not taking the pill.

Professor Strassmann then compared the total levels of estrogen and progesterone in these women to the total levels of synthetic hormones, progestin and estradiol, in women taking one of several commonly prescribed birth control pills for 28 days.

That information was taken from the package inserts for each contraceptive formulation.

'It is critically important to know whether hormonal contraception further exacerbates this risk,' Professor Straussman said.

In American women, breast cancer is the most common type of cancer and the second leading cause of death.

Evidence shows that a prolonged period of high levels of estrogen, for example during pregnancy or in obese women, can cause abnormal cells to grow.