the reason some women gain lots of weight during pregnancy is the same reason some women gain on the pill. For example women some times retain a lot of water during pregnancy (swollen ankles, puffy face, lots of bloating) when taking birth control you are putting artificial pregnancy hormones in your body some women react to this with lots of water retention up to 15Ibs. I personally gained about 20Ibs on the pill ortho tri-cyclen over the coarse of two months and I can assure you I tried very hard to get it off. My doctor recognized my weight gain as water retention because I had swollen ankles, puffy face and even my rings and shoes were becoming too tight. I changed pills to Yasmin which has a diuretic in it and began taking 100mgs of spironolactone (a diuretic). Within a month I lost all the weight and went back to normal. All birth control pills have the potential to cause this kind of water retention it really depends on the person, the problem is most doctor dont take the time to treat patients who react to birth control this way. If you do react to the pill with weight gain switch to a different pill and try to cut down on salt and drink enough water to keep you hydrated, your doctor may or may not perscribe a diuretic depending if its right for you.