I just had an appointment with the OB/GYN. You can get pregnant from ovulation on even during your period, if the egg is still in the womb and has not been shed yet with the uterine lining. There is little protection a day or two into your period from bc pills but it is an unlikely time, especially later in the week, for conception to occur as the lining and egg are shed but it is possible if the egg has not shed yet.

If there were the same hormones during this week as are in the other 3 weeks, your body wouldn't know to have a period. The week of your period these are placebo pills, no hormones, only reminders to continue taking them daily. There are some conditions, such as endometriosis and cysts that you do take hormone pills consecutively, skipping the placebo week, remaining on the hormones and so you do not have a period.

BC pills taken regularly, at the same time daily (without interference from missed pills, other medications or additional stressors on bodily hormones) are very effective in preventing pregnancy.