In May, after I reported that birth control pills are less effective for overweight women, a reader e-mailed me, basically saying, “OK, I’m obese. But do I really have to switch from the pill to something else? If so, what would work better?” I called ob/gyn Julie Laifer, MD, an attending physician at Bridgeport Hospital–Yale New Haven Health and a member of the HealthyWoman from Bottom Line advisory board, to discuss the matter.

Dr. Laifer told me that this reader and other women in the same situation should indeed consider another form of birth control, given that about 2% to 4% of obese women on the pill end up pregnant in a year’s time, compared with just 1% of normal-weight women. Why the discrepancy? The pill contains hormones that halt ovulation—but extra fat tissue causes these hormones to be metabolized faster, so they don’t stay in a heavy woman’s system as long as they should… and heavier women have more blood volume, so the hormones get diluted slightly. The same problems with efficacy apply to the contraceptive skin patch, another type of hormonal birth control.

An added concern is that both the pill and patch may cause weight gain—an undesirable side effect for most women, of course, but particularly for those who are already overweight. Plus, obesity increases the risk for blood clots—and the estrogen in the pill further augments this risk. Why not boost the effectiveness of hormonal contraceptives by giving heavy women a larger dose? Because as dosage rises, so do the risks.

Better bets for obese women who want temporary birth control…

Consider the ParaGard brand of intrauterine device (IUD), which contains no hormones (the other brand available in the US, Mirena, does contain hormones). ParaGard is a small, T-shaped device made of copper and plastic that a doctor inserts into the uterus via the vagina. It prevents pregnancy by interfering with sperm movement, and its success rate of 99.4% is not compromised by body weight. Downsides: An IUD can cause heavier, longer periods… and carries a small risk for uterine perforation. (While diaphragms, spermicidal jellies and condoms also are unaffected by weight and have minimal risks, Dr. Laifer cautioned that they are far less effective at preventing pregnancy.)

Permanent contraception options…

Tubal ligation, in which the Fallopian tubes are surgically cut or blocked, is very effective. However, it is not perfect—it has a failure rate of 0.5% during the first year and 1.5% to 2% after 10 years because occasionally the area heals improperly or other problems develop that impair effectiveness. Tubal ligation carries the same risks as any major surgery, and the procedure is more difficult and takes longer in obese patients.

Less invasive are two new options, the products Essure and Adiana, which require no incisions and can be done in a doctor’s office using local anesthesia. The gynecologist inserts a scope through the vagina and into the uterus. Then a small device—a metal spring about an inch long for Essure, or a soft piece of silicone the size of a grain of rice for Adiana—is placed into each Fallopian tube. Over the next three months, scar tissue forms and permanently blocks the tubes so sperm and eggs cannot get through. These methods have a failure rate of 0.3% to 1.6%, which is better than the failure rate of the pill for obese women. Because these procedures are relatively new, there is limited data on their long-term effects—so be sure to discuss their benefits and risks with your doctor.