By Sipokazi Maposa
Obese and overweight women who use hormonal contraceptives have a greater risk of becoming pregnant than lean women with a lower body mass index (BMI), according to international researchers.
Body weight addresses overall body size, while BMI reflects fat.
In an article published in the Cochrane Library, the researchers cautioned that metabolic changes caused by obesity and greater body mass could reduce the effectiveness of hormonal contraceptives such as the Pill, skin patches, vaginal rings, implants and injectables.
Reviewing previous research and clinical trials involving nearly 40 000 women, one of three studies using BMI found a higher pregnancy risk for overweight or obese women.
In a trial that looked at two types of birth control pills, women with a higher BMI were more susceptible to falling pregnant.
Among skin patch users, more pregnancies were found among women weighing 80kg or more. They also found that body size may not affect the efficacy of injectable or implanted contraceptives.
Generally, being overweight or obese may affect how well hormonal birth control worked to prevent pregnancy, the researchers concluded.
"Counselling women about how well birth control methods work is hard, with little information on any one method. The field could use randomised trials of birth control methods with the groups divided by BMI. However, the birth control methods studied here are among the most effective when the directions are followed," researchers wrote.