Scientific studies indicate that hormonal birth control doesn't directly cause weight gain, according to MayoClinic.com. Still, many women who use it do gain weight -- particularly in their hips, thighs and breasts. This could be a result of hormones that increase hunger, lead to water retention and plump up your existing fat cells.
Use an online calorie calculator like the one available at MayoClinic.com to get an estimate of the number of calories you need to take in each day to avoid gaining weight.
Write down the total number of calories in everything you eat each day. Keep your calorie count at or below the number of calories required to maintain your weight.
Keep plenty of healthy snacks on hand, such as fresh fruit and vegetables and low-fat dairy products, to help you adjust to your increased appetite.
Get plenty of exercise to burn calories, control appetite and reduce water weight. Try to exercise at least 30 minutes most days of the week.
Increase your fluid intake to help prevent water retention caused by hormonal changes. Reduce your sodium intake to 1,500 mg or less each day to further help reduce and prevent water weight gain.
Talk to your doctor about other forms of birth control if you have trouble controlling weight gain even with a healthy diet and exercise plan.