There's no gaurentees.. usually your most fertile time will be approx mid way between your periods.
You can find out exactley when a few ways.. one measure your morning body temp (when you first wake up) and watch for a very slight rise.. You can watch your discharges etc....
Also..There are menstral planners available on the net you can use for free which I find really usefull (http://www.mymonthlycycles.com) they will help you work out your times more exactley.
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