In order to conceive, you need to have unprotected sex at the right time of the month, ie during your most fertile period, which is ovulation, or when an ovary releases an ova, or egg.  Remember that a cycle is measured from the first day of your period, to the first day of your next period.  In a normal 28-day cycle, ovulation takes place around the 14th day, in a 35-day cycle, it is around the 21st day, but fertilisation can take place a few days beforehand.  Your fertile period usually starts four days before ovulation, ending 24 hours afterwards.