News

Attention, ladies: You can't blame those extra pounds on the Pill.

New research concludes that the common belief that taking birth control pills leads to weight gain is unfounded.

Rather, women using oral contraceptives are typically in an age group in which weight gain is common, and oral contraceptives serve as a convenient excuse.

"This review found no evidence to support a causal association between contraceptives and weight gain," said Laureen Lopez, an epidemiologist who conducted the research for Family Health International, a non-profit group based in Research Triangle Park, N.C.

She said that women and men put on weight as they age, but over time, women have come to blame oral contraceptives, an erroneous view often bolstered by their doctors.

"Over time, many people do put on weight, and some women's perception about weight gain may be that it's because of the Pill," Dr. Lopez said.

The study, published in the Cochrane Library, is a meta-analysis, a compilation and analysis of research that has already been published. Dr. Lopez and her team looked at 44 randomized controlled trials that compared the efficacy of different forms of hormonal contraceptives.

Only three of the studies made a comparison between a hormonal contraceptive and an inactive placebo. None showed a significant difference in the weight of women using a contraceptive to those using a placebo.

In Canada, almost half of sexually active women of reproductive age take a hormonal contraceptive in pill, patch or injection form.

Most birth control pills contain two hormones, estrogen and progestin. The Pill works in three different ways to prevent conception: Estrogen tricks the body into thinking it is already pregnant so ovulation does not occur; estrogen also causes changes in the uterine tissue so that it is inhospitable to an egg; and progesterone causes the cervical mucous to thicken, preventing sperm from passing to the uterus and Fallopian tubes.

Fear of weight gain is one of the most common reasons that women don't use -- or discontinue use of -- hormonal contraceptives. In a survey of women 18 to 45, published in the International Journal of Gynecology and Obstetrics, half believed that oral contraceptives caused weight gain, and one in five women said the fear of gaining weight was the principal reason they would not start or would discontinue using them.

"The myth that the Pill causes weight gain is deeply entrenched," Carolyn Westhoff, a professor of obstetrics and gynecology at Columbia University, said.

"The most weight-obsessed segment of the female population just won't take hormonal contraception at all because they think they're going to gain weight."

If used correctly and consistently, hormonal contraceptives are more than 99-per-cent effective in preventing pregnancy. Practically speaking, however, their effectiveness is estimated at 92 per cent.

While the principal role of hormonal contraceptives is to prevent pregnancy, there are other health effects, such as slightly increased risk of blood clots, heart attack and liver cancer, as well as decreased libido. Hormonal contraceptives do not prevent sexually transmitted diseases. Women who take them do have a reduced risk of ovarian and endometrial cancer.