There are several reasons why overweight and obese women may not want to take a hormonal birth-control. It may be for health reasons, like furthering the risks of high blood pressure, heart disease, or blood clots. Many decline to take birth control for religious reasons. Natural Family Planning, or NFP, has recently gained momentum, particularly among Catholics. NFP is the only form of birth control condoned by the Roman Catholic Church other than abstinence.
Further, the birth control pill is much less effective at preventing pregnancies in overweight and obese women. A study conducted in Washington state found that the risk of unwanted pregnancy among women taking the pill to be 60 percent higher among overweight women, and 70 percent higher among obese women. Other studies suggested that the same thing may also be true of hormone-based birth control, such as implants or the patch.
Religious reasons aside, some women who cannot take the pill for weight-related health reasons may consider turning to Natural Family Planning, also known as the rhythm method and fertility awareness. NFP includes knowledge of one’s menstrual cycle(calendar method), tracking one’s basal body temperature (BBT), and observing cervical mucus (Billings method). These are all indicators of when one is ovulating. However, the rhythm method is only about 75 to 87 percent effective (WebMD). That means there’s a one-in-four chance of pregnancy every time you have sex without another form of birth control.
There’s another issue to consider when it comes to obesity and the rhythm method. Women who are obese are more likely to have irregular periods and hormone cycles. This may make it much harder for overweight women to predict when they are ovulating.
If you are overweight and it is important to you that you to not get pregnant, you may want to consider a barrier method of birth control, such as condoms, diaphragms, or sponges.