Contraception may be less effective in overweight or obese women, according to an article in the Boston Globe. However, more studies are necessary to confirm this suspicion.

Many studies of contraceptive efficacy have excluded clinical trials on overweight or obese women, according to an article on uptodate.com. The doctors also concluded that the pill, the patch and contraceptive implants yield a higher failure rate in obese women.

The study also noted that “even if the risk of contraceptive failure is increased, the effectiveness probably remains relatively high.”

Dr. Alan Calhoun, the medical director of University Health Services at the University of Massachusetts, shares the same sentiment.

“Birth control pills are still very effective for women who are overweight,” Calhoun said.

Calhoun recommends oral contraceptives [pills] and intrauterine devices [IUDs], which are small, plastic, t-shaped devices that are inserted into the uterus.

Calhoun also said that pills are the most commonly utilized form of birth control among young women, and that most of the pills are made of the hormones estrogen and progesterone.

Estrogen helps develop the uterus and stimulate ovulation while progesterone prepares the uterus for a fertilized egg. These hormones prepare the female body for pregnancy.

“Being on a birth control pill, from your body’s hormonal level, is like telling it that it’s already pregnant, so it doesn’t need to ovulate,” Calhoun said.

Some contraceptives – such as Depo-Provera, a tri-monthly shot – are solely progesterone. Implanon – a little plastic device that gets buried into the arm and reinserted every five years – also falls under that category.

“Traditionally all of these products are available for overweight women,” Calhoun said.

The problem may be that overweight women aren’t getting an adequate amount of the hormones that come in standardized products, such as Implanon, according to the study.

“When you have a larger person, you have more body volume and the hormone has to spread around over a greater mass of tissue,” said Calhoun. “But it’s more complicated than that … overweight women metabolize differently than underweight women might.”

However, Calhoun asserted that “most of the time it doesn’t matter that much.”

“The concern arises when women become pregnant with an inadequate dose of medication,” he said.

Other problems could arise from bad timing. In order to prevent ovulation, the hormone levels need to rise soon enough, according to the study.

In an average-weight woman, it might take five days to reach to a “steady state” level, but 10 days for an obese woman to reach that same level. Ovulation normally occurs on day 14, according to Calhoun.

“Day 10 might be adequate, but if it’s delayed, then you ovulate, and then the pill has failed,” Calhoun said.

A measure of a woman’s Body Mass Index, or BMI is used to determine if she is overweight. BMI calculations are based on a height and weight ratio.

“It’s not a perfect analysis of body fat composition … but it’s a better scale than just weighing people because it adjusts for height,” said Calhoun.

A normal BMI is between 20 and 25, overweight is 25-30, obese is 30-35, and extremely obese is over 35. In the United States, over one third of the population has a BMI over 30.

Calhoun also referenced the political debate over contraceptives, which Republicans are fighting to exclude from free healthcare for religious reasons.

“If women work for a Catholic institution … and they’re not Catholic and they want to have free contraception, who gets to choose? Do they get excluded from this benefit that the rest of the women in the United States are getting?” Calhoun asked.

Besides preventing pregnancy, contraceptives also decrease healthcare costs because unplanned pregnancies are extremely expensive, according to Calhoun. Birth control in young women can also decrease the risk of uterine cancer, and possibly breast cancer. But estrogen supplements may actually increase the incidence of breast cancer as women go through menopause.

Overall, Dr. Calhoun believes that birth control is safe for women of all sizes.

“I wouldn’t want someone to be frightened that all of a sudden they’re not being protected,” he said.

Mary Reines can be reached at [email protected].