Hormonal Contraceptives May Not Be as Effective in Overweight or Obese Women

July 14, 2010 — Hormonal contraceptives may not be effective for contraception in overweight or obese women, according to the results of a systematic review reported online July 7 in the Cochrane Database of Systematic Reviews.

"Obesity has reached epidemic proportions around the world," write Laureen M. Lopez, from FHI in Research Triangle Park, North Carolina, and colleagues. "Metabolic changes in obesity and greater body mass may lead to reduced effectiveness of hormonal contraceptives, such as the skin patch, vaginal ring, implants, and injectables. We systematically reviewed the evidence on the effectiveness of hormonal contraceptives among overweight and obese women."

The reviewers searched MEDLINE, CENTRAL, POPLINE, EMBASE, ClinicalTrials.gov, and ICTRP, and they also asked investigators of the retrieved studies to identify other trials. Inclusion criteria were all studies of any design evaluating any type of hormonal contraceptive, with pregnancy as the primary outcome and identification of overweight or obese women by an analysis cutoff point for weight or body mass index (BMI; kg/m²).

Two review authors abstracted data and included life-table rates where available. Odds ratios (ORs) with 95% confidence intervals (CIs) were calculated for dichotomous variables, primarily comparing overweight or obese women vs women of lower weight or BMI.

There were 7 reports identified of 11 trials enrolling a total of 39,531 women. Pregnancy risk for overweight or obese women was higher in 1 of 3 studies using BMI. Compared with women with a BMI of less than 25 kg/m², women with a BMI of 25 kg/m² or more had higher risk for pregnancy in this trial of 2 combination oral contraceptives (OR, 1.91; 95% CI, 1.01 - 3.61).

Body weight, but not BMI, was associated with pregnancy among women using the skin patch (reported P < .001). Women weighing 70 kg or more had higher pregnancy rates than those weighing less than 70 kg in studies of a never-marketed vaginal ring (P < .0013) and a 6-rod implant (P < .05). In contrast, there was no trend by body weight in 2 implant studies, and no pregnancies occurred in trials of an injectable contraceptive.

"Body weight addresses overall body size, while BMI generally reflects the amount of fat," the review authors write. "Only one of three studies using BMI found a higher pregnancy risk for overweight women. The efficacy of implants and injectable contraceptives may be unaffected by body mass."

Limitations of this review include lack of randomization in all studies and incomplete data and loss to follow-up in some studies.

"The field could use trials of contraceptive methods with groups stratified by BMI," the review authors conclude. "The current evidence on effectiveness by BMI is limited. However, the contraceptive methods examined here are still among the most effective when the recommended regimen is followed."

The review authors have disclosed no relevant financial relationships.

Cochrane Database Syst Rev. Published online July 7, 2010. Abstract

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