Birth Control Pill's Effectiveness in Overweight Women Questioned

1. Implanon implant may not work as effectively. [Dr. Melissa] Gilliam, [board president of the Society of Family Planning], conducted a small study measuring how well Implanon -- a three-year implant inserted beneath the skin of the arm -- works in obese women; she found that blood levels of etonogestrel, released by the implant to prevent ovulation, were markedly lower in obese women compared to women who weren't overweight.

"We don't want to say this reflects clinical efficacy since the hormone levels were still high enough to prevent ovulation," said Gilliam. "But this is the first study to look at Implanon in obese women and it gives us some indication that it behaves differently. We need more information on this from larger studies."

2. Later hormone peak for pill. Other researchers examined the more commonly used oral contraceptives and found that obese women who take the pill, on average, don't reach a prime hormone level for preventing ovulation until day 10 of their pill pack, compared to women at a healthy weight who reach this hormone level by day 5. Whether this increases an obese woman's chances of becoming pregnant isn't known, said Dr. Alison Edelman, an associate professor of obstetrics and gynecology at Oregon Health & Science University who led the study.

"Some studies do show a slight reduction in contraceptive effectiveness among obese women," Edelman said. "But it's a little too early to say that they need to use a different method of birth control." Obese women may, though, want to speak to their doctors about the research findings to see whether they should take a birth control pill continuously or whether they should switch to an implant or intrauterine device to avoid having the dip in hormone levels.

3. Avoiding weight gain from Depo Provera shot. Overweight women, in particular, are looking for a contraceptive method that won't cause weight gain. And Depo Provera -- a hormone shot given once every three months -- has been known to cause significant weight gain in 1 in 4 women. The trouble is doctors don't know how to predict which women will gain those 10 pounds or more.