If you’re packing excess pounds, they might diminish your birth control pill’s efficacy, according to a new study from Portland’s Oregon Health & Science University that was reported on by Reuters. Why? Oral contraceptives act differently in bodies of obese women than in normal weight women.

For the study, 20 women ages 18 to 35 (who weren’t already using the pill) took oral contraceptives containing ethinyl estradiol (synthetic estrogen) and levonorgestrel (synthetic progesterone) for two cycles. Half the participants were obese; the remaining women were of normal weight.

In obese women, levonorgestrel levels took five days longer to suppress ovulation than in normal weight women. (For one obese participant, it took 20 days to reach an adequate ovulation suppression level.)

The lag time to reach steady levels of levonorgestrel might present a “window of opportunity” for the ovary to prepare to release an egg, said Alison B. Edelman, MD, MPH.

The study’s results are significant, but Edelman and her team said that “additional studies are needed before we recommend a change in clinical practice in regard to use of oral contraception in obese women.”

Find out how the pill may change the dating game here.