Susan A. Ballagh, MD, Visiting Associate Professor, Harbor-UCLA Medical Center, Torrance, CA
Before we get into the issue you ask about, I want to address your prolonged, heavy bleeding. Menstrual bleeding rarely continues beyond seven days, with the amount of bleeding diminishing throughout that time. I strongly recommend you contact your health care provider for a complete examination.
Now, as for your question about when to start taking oral contraceptives, there's really no hard-and-fast rule about when to start taking birth control pills. Starting in the first few days of your cycle is the surest way to prevent inadvertently taking pills while you are pregnant. If there is any chance you might be pregnant (you feel fatigue, you had to take emergency contraception last cycle, etc.), you can always check a home pregnancy test. Alternatively, you can start on the Sunday after your period starts which provides a point-in-time for when a new pill pack should begin, and, unless you're taking continuous oral contraceptives, ensures that you don't have a period on the weekend (always a nice benefit!). Some providers even suggest that you start pills as soon as you get them, provided you are reasonably sure that you are not likely to get pregnant this current cycle.
However, we still recommend you use a backup form of birth control during the first month of oral contraceptive use just to be on the safe side. This could be a condom, diaphragm, the Today sponge, etc.
What's most important about oral contraceptive timing is that you take your pill at the same time every day. This helps you remember to take it (i.e., you always take your pill after you brush your teeth) and helps maintain steady hormone levels. It is also important that you don't miss a day. If you do miss a day, take two pills the following day. If you miss two pills, take two pills a day for two days. But if you miss more than that, you should use an alternative form of birth control until you start another pill pack. You should also check with your health care professional as to whether you should continue the current pack or just start over again once your period begins.