Concerns about safety have led to making birth control pills with less of the hormone estrogen. Pills with less estrogen might not work as well to prevent pregnancy and could cause bleeding problems. This review looked at studies that compared pills with 20 µg ethinyl estradiol versus pills that have more estrogen.
In July 2013, we did computer searches for randomized trials of pills with 20 µg estrogen versus more estrogen. We did not find any new trials. For the initial review, we also wrote to researchers and makers of birth control pills to find other trials.
Studies had to be written in the English language, include at least three cycles, and focus on birth control. The trials had to report on pregnancy, bleeding problems, or stopping the pills early. We also looked at side effects.
More women taking the pills with less estrogen quit the studies early. The women on less estrogen also had more bleeding problems than those taking pills with more estrogen. Pregnancy rates seemed to be the same between groups, but the studies may not have been large enough to know for sure. This review did not focus on the rare events needed to test whether birth control pills with 20 µg estrogen were safer. Also, most trials compared pills with different types of the hormone progestin, which could also affect bleeding patterns. The high losses in many trials make the results hard to interpret.