The first day of your cycle is the first day of your period, most woman ovulate some where around day 14, then they start their cycle again around day 28. This by no means is birth control though because you could ovulate at any time depending on how regular your cycle is. If you really want to know exactly, or very close to googling basel temperature charting, and fertile mucus. That will help you understand you own body.
at 5:46 PM on Jul. 4, 2008