birth control without gaining weight?

okay, so I'm a heavier girl. I'm wanting to be on birth control, but I definitely /do not/ want something that'll make me gain weight, because I already have trouble losing weight. I just always get so paranoid when my fwb and I have sex that I'm going to get pregnant. my periods are already hella irregular, so it's scary to think I'm late, especially if I've had sex recently. any suggestions on birth control?