A:

Many factors affect how well the birth control pill will work and how safe it will be. Obesity probably affects both.

We don't know how great the effects of obesity are because research studies haven't told us. Many studies did not include overweight women. Other studies did not take into account all other important factors such as pill dose, proper use of the pill, the fertility of the women and how often they had sex.

Obese and overweight women using the pill to prevent pregnancy have a slightly higher failure rate compared with normal weight women. However, the overall effectiveness of the pill is still very high in overweight and obese women when used correctly. And the pill is much better at preventing pregnancy than some other contraceptives, such as condoms.

A few points to keep in mind:

• Pregnancy is more risky in obese women compared with normal weight women. So consider that when looking at the risks and benefits of using the pill if you are overweight.
• Obese women do not appear to have a higher risk of medical complications when they use hormone contraceptives that contain only progesterone.
• Obesity and birth control pills that contain estrogen both increase the risk of blood clots. Extra care is needed to watch for signs of this complication when an overweight woman uses the pill.

You should review all the issues with your doctor to decide if the birth control pill is the best choice for you.