According to the lead author Alison Edelman, M.D., “Issues surrounding weight are hard to study in humans, and the research thus far has been insufficient to demonstrate whether or not oral contraceptives cause weight gain or loss. But this is an extremely important question as concern about weight gain is one of the main reasons why women may avoid or discontinue birth control, which in turn places them at greater risk for an unplanned pregnancy."
For the study, the monkeys were given oral contraceptive doses based on their body weight and were treated for a course of eight months. They collected information on the animals’ nutritional intake, body fat mass, and lean muscle mass. They found that the animals with average body weights stayed the same weight, and that the obese monkeys actually lost 8.5% of their body weight. In other words, heavier women could potentially lose weight while on the pill.
So if you’re concerned about taking birth control pills and having that cause you to lose weight, your fears are unfounded. Of course, all decisions about the right choices for birth control should be made by women in conjunction with their doctors.
"Combined oral contraceptives and body weight: do oral contraceptives cause weight gain? A primate model"
A. Edelman, J.T. Jensen, M. Bulechowsky and J. Cameron
Hum. Reprod. (2011) 26 (2): 330-336. doi: 10.1093/humrep/deq335