Actually, there is no reason why, in spite of the common belief. But it is true that, though rarely, some women do gain a little weight after starting taking oral contraceptives. It is due to fluid retention, which means that it is just a temporary side effect.
Does family planning cause weight gain?
By Zaira Salvador B.Sc., M.Sc. (embryologist).
Last Update: 12/28/2017
Bachelor's Degree in Biotechnology from the Technical University of Valencia (UPV). Biotechnology Degree from the National University of Ireland en Galway (NUIG) and embryologist specializing in Assisted Reproduction, with a Master's Degree in Biotechnology of Human Reproduction from the University of Valencia (UV) and the Valencian Infertility Institute (IVI)