Getting periods totally depend on your hormones and it's always better not to meddle with it by taking tablets which are also hormones. So better avoid it for your own good.
The safest time could be first 8 days and after 18 days if cycles are regular... if cycles are irregular it won't work... you have to work it out very carefully for the days...Between 8- 18 days are very fertile period. chances are more to become pregnant... this method what we call as calender method though effective it is not 100% contraception
... Hope this helps.