Re: how to calculate unsafe periods
The unsafe period is around ovulation, which comes abut 14 days before your next period.
If you have very very regular periods, you can calculate when this might be and then avoid all sex for the days before and after. If you don't have regular periods you'll need a time machine, start your period, then go back 18 days and then make sure you don't have sex for the next week.
Basically, this is the rhythm method and has a ridiculously high failure rate. It should only be used by people who don't mind if they do end up pregnant.