Hi,
Welcome to ehealthforum.
When you start ovulating (with egg being released from ovary in your cycles), then usually you tend to have less bleeding as compared to the anovulatory cycles (without egg release from ovary).
Before starting on birth control pills, request your treating gynecologist to go ahead with hormonal analysis to see if they are fine and not the cause for prolonged bleeding. If any abnormalities are detected, then treatment will be directed to resolve the problem.
Take care.