Wow, from changes in insulin levels to water retention, it seems that there has historically been a correlation between oral contraceptives and weight gain. The newer, lower-estrogen pills should provide relief from many of these issues.

An excellent article that discusses weight gain and birth control pills. Some women are afraid of taking the pill because of weight gain and I like that the article points out that most women shouldn't gain anymore than 5% of their body weight when starting the pill. If you do you should see your doctor about insulin resistance.