Half the women were obese and the other half were of normal weight.
For the obese women, it took about ten days for the hormones in the pill to start suppressing ovulation.
It took about five days for the normal weight women.
Researchers admit that findings of studies on body mass index and the effectiveness of thebirth control pill have been inconsistent.
They are now calling for more studies before any recommendation is made on contraceptive use.
The study is published on Contraception, online June 4, 2009.