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August 3, 2010

Hormonal Contraceptives are Less Effective in Overweight/Obese Women

A systematic review of results reported online has concluded that hormonal contraceptives (such as skin patches, vaginal rings, implants, pills, and injections) may not be as effective in overweight and obese women.

With obesity rates increasing worldwide every year, it is important that these findings be shared with the public.

The results from seven studies on 39,531 women on the success of hormonal contraceptives were examined and the results were varied among the different methods of birth control.  Some methods such as the birth control pill and the patch were less effective at preventing pregnancy in the obese and overweight women who used a hormonal contraceptive.

Body mass index, or BMI, played a large role in the effectiveness of the birth control pill.  Women with a BMI higher than 25 (classified as overweight), were more likely to get pregnant while using the pill than women with a BMI under 25.

Body weight, on the other hand, was the factor among women using the skin patch, vaginal ring, implant, or injection.  The body weight due to fat on the body determined the pregnancy risk among women using contraception.  Overweight and obese women using the skin patch, progestin implant, and vaginal ring were more likely to get pregnant than women of normal weight using these forms of contraception.  Curiously enough, the effectiveness of the hormonal injection was not affected by BMI or body weight.  One possible reason for the decreased effectiveness of the skin patch is simply that there is a higher density of fat cells in areas where the patch is to be placed.  The fat cells in the way of the blood stream can absorb and store the hormones preventing them from reaching the blood stream.

The exact reason why overweight and obese women are at a greater risk of pregnancy while using hormonal contraceptives is unknown.  Although several theories imply that it has something to do with specific biological factors.

Liver Enzymes – overweight women can experience an increased regulation of liver metabolism.  These higher levels of enzymes can help break down the hormones in oral contraceptives.  Heavier women have a greater circulating blood volume and body mass therefore there is more tissue for blood to circulate though. The chance that enzymes will break down the hormones in contraception is faster therefore the levels of circulating hormones can be decreased.

Hormone Storage – Hormones like estrogen and progestin that are found in birth control are stored in body fat.  The more fat cells on a woman, the greater her chances of the hormones becoming stored in her fat rather than flowing into her bloodstream.

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