Weight gain is one of the unwanted side effects of some oral contraceptive pills. This is the result of water retention rather than the accumulation of fat. The more premium oral contraceptive pills that contain progesterone in the form of drospirenone will not cause bloating or weight gain.
This is because drospirenone acts like natural progesterone and counteracts the mechanism that causes water retention and therefore bloating. Examples of oral contraceptives which contain drospirenone include Yasmin and Yaz. In addition to their contraceptive effects, they will regulate your menses for you.