With spring upon us, countless women will start shopping for that perfect summer bikini. And apparently many will be unaware that whatever size they were last summer has jumped a notch or two … or even three … this season.

New research shows that while women may not want to put on the pounds, a considerable number are not aware of the increase when they do gain weight.

In a first-of-its-kind study, investigators surveyed women’s perception of weight gain and also looked for any possible correlation to birth control use. They analyzed the weight gain of almost 500 over a three-year period. About one-third of these women used a birth control shot, while another third used oral contraceptives and one-quarter relied on non-hormonal birth control methods.

Every six months the women were given a questionnaire that asked them if they felt they had gained weight.

The findings indicated that of those who gained about five pounds over a six month period, almost one third of the subjects had no idea of the additional pounds put on. One quarter of the women who had gained nearly 10 pounds also had no clue. However, black women, as well as those using the birth control shot, were significantly more conscious of their weight gain. This may be because weight gain is a potential side effect of the shot, so women who get it could be paying more attention to weight fluctuations.

Being mindful of your weight is important to staving off potential health problems like heart disease and diabetes, so the researchers offer one simple tip to women: Weigh yourself. Stepping on the scale regularly and keeping track of your weight can help you keep a two-pound gain from turning into a 20-pound one.