We cannot predict anybodies biological fertile time based on general advice. I am 23 years old and sometimes when I have my period, even a few days into it, I feel very tired. I have a question for you. Pregnancy occurs when a woman has sexual intercourse during her fertile time, which is when she is ovulating. Go to Physically Speaking for a more in depth description of how ovulation occurs.
Ovulation can potentially occur twice in a womans cycle.
Usually it occurs midcycle,in response to the action of hormones, but it can also happen spontaneously at another time in response to the action of the moon. So you really need to know when you have fertile mucus and when your natal lunar return occurs. This is just a general rule, It is impossible to say that on day X ovulation does occur, because it is a highly individual process, for example one woman may ovulate on day 12 and another on day 15. There are no hard and fast rules, menstrual cycles vary from woman to woman and even cycle to cycle. The rhythm method of counting the days can be dangerous because of this variation.