Hormones for birth control in overweight or obese women

Excess body weight has become a health problem around the world. Being overweight or obese may affect how well some birth control methods work to prevent pregnancy. Hormonal birth control includes pills, the skin patch, the vaginal ring, implants, injectables, and hormonal intrauterine contraception (IUC).

Until 4 August 2016, we did computer searches for studies of hormonal birth control among women who were overweight or obese. We looked for studies that compared overweight or obese women with women of normal weight or body mass index (BMI). The formula for BMI is weight (kg) / height (m)2. We included all study designs. For the original review, we wrote to investigators to find other studies we might have missed.

With 8 studies added in this update, we had 17 with a total of 63,813 women. We focus here on 12 studies with high, moderate, or low quality results. Most did not show more pregnancies for overweight or obese women. Two of five studies using birth control pills found differences between BMI groups. In one, overweight women had a higher pregnancy risk. The other found a lower pregnancy rate for obese women versus nonobese women. The second study also tested a new skin patch. Obese women in the patch group had a higher pregnancy rate. Of five implant studies, two showed differences among weight groups. They studied the older six-capsule implant. One study showed a higher pregnancy rate in years 6 and 7 combined for women weighing 70 kg or more. The other reported pregnancy differences in year 5 among the lower weight groups only. Results for other methods of birth control did not show overweight or obesity related to pregnancy rate. Those methods included an injectable, hormonal IUC, and the two-rod and single-rod implants.

These studies generally did not show an association of BMI or weight with the effect of hormonal methods. We found few studies for most methods. Studies using BMI rather than weight can show whether body fat is related to how well birth control prevents pregnancy. The methods studied here work very well when used according to directions. The overall study quality was low for this review, especially in the older reports. However, many studies would have higher quality for their original purpose than for the comparisons here.

Authors' conclusions: 

The evidence generally did not indicate an association between higher BMI or weight and effectiveness of hormonal contraceptives. However, we found few studies for most contraceptive methods. Studies using BMI, rather than weight alone, can provide information about whether body composition is related to contraceptive effectiveness. The contraceptive methods examined here are among the most effective when used according to the recommended regimen.

We considered the overall quality of evidence to be low for the objectives of this review. More recent reports provided evidence of varying quality, while the quality was generally low for older studies. For many trials the quality would be higher for their original purpose rather than the non-randomized comparisons here. Investigators should consider adjusting for potential confounding related to BMI or contraceptive effectiveness. Newer studies included a greater proportion of overweight or obese women, which helps in examining effectiveness and side effects of hormonal contraceptives within those groups.

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Background: 

Obesity has reached epidemic proportions around the world. Effectiveness of hormonal contraceptives may be related to metabolic changes in obesity or to greater body mass or body fat. Hormonal contraceptives include oral contraceptives (OCs), injectables, implants, hormonal intrauterine contraception (IUC), the transdermal patch, and the vaginal ring. Given the prevalence of overweight and obesity, the public health impact of any effect on contraceptive efficacy could be substantial.

Objectives: 

To examine the effectiveness of hormonal contraceptives in preventing pregnancy among women who are overweight or obese versus women with a lower body mass index (BMI) or weight.

Search strategy: 

Until 4 August 2016, we searched for studies in PubMed (MEDLINE), CENTRAL, POPLINE, Web of Science, ClinicalTrials.gov, and ICTRP. We examined reference lists of pertinent articles to identify other studies. For the initial review, we wrote to investigators to find additional published or unpublished studies.

Selection criteria: 

All study designs were eligible. The study could have examined any type of hormonal contraceptive. Reports had to contain information on the specific contraceptive methods used. The primary outcome was pregnancy. Overweight or obese women must have been identified by an analysis cutoff for weight or BMI (kg/m2).

Data collection and analysis: 

Two authors independently extracted the data. One entered the data into RevMan and a second verified accuracy. The main comparisons were between overweight or obese women and women of lower weight or BMI. We examined the quality of evidence using the Newcastle-Ottawa Quality Assessment Scale. Where available, we included life-table rates. We also used unadjusted pregnancy rates, relative risk (RR), or rate ratio when those were the only results provided. For dichotomous variables, we computed an odds ratio with 95% confidence interval (CI).

Main results: 

With 8 studies added in this update, 17 met our inclusion criteria and had a total of 63,813 women. We focus here on 12 studies that provided high, moderate, or low quality evidence. Most did not show a higher pregnancy risk among overweight or obese women. Of five COC studies, two found BMI to be associated with pregnancy but in different directions. With an OC containing norethindrone acetate and ethinyl estradiol (EE), pregnancy risk was higher for overweight women, i.e. with BMI ≥ 25 versus those with BMI < 25 (reported relative risk 2.49, 95% CI 1.01 to 6.13). In contrast, a trial using an OC with levonorgestrel and EE reported a Pearl Index of 0 for obese women (BMI ≥ 30) versus 5.59 for nonobese women (BMI < 30). The same trial tested a transdermal patch containing levonorgestrel and EE. Within the patch group, obese women in the "treatment-compliant" subgroup had a higher reported Pearl Index than nonobese women (4.63 versus 2.15). Of five implant studies, two that examined the six-capsule levonorgestrel implant showed differences in pregnancy by weight. One study showed higher weight was associated with higher pregnancy rate in years 6 and 7 combined (reported P < 0.05). In the other, pregnancy rates differed in year 5 among the lower weight groups only (reported P < 0.01) and did not involve women weighing 70 kg or more.

Analysis of data from other contraceptive methods indicated no association of pregnancy with overweight or obesity. These included depot medroxyprogesterone acetate (subcutaneous), levonorgestrel IUC, the two-rod levonorgestrel implant, and the etonogestrel implant.

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