don't worry about messing up the charts. a simple way to do it is a person ovulates at 14 days before their next period. so chart your periods. the first day of your period is day one. for example a persons cycle may look like - september 26 is day one in the cyle. Keep counting the days until the next period starts on october 23rd. and again on november 19th. two months in a row the person had a 28 day cycle

so now that the cycle is known count forwards to the next period (december 16). from december 16 count backwards 14 days and that is when the person ovulates. in this case its december 2nd. having sex anytime on the 1st, 2nd and 3rd (because sperm is only viable for 72 hours) will result in prgnancy.

My son is happy proof of this and I was 30 when my boyfriend and I visited my ob/gyn for this information.

I also use this concept too as birth control in on those three fertile days the person does not have sex. and have been successful for 12 years.

Good luck