Approximately every 28-32 days an egg is released into the Fallopian tube, and it takes several days after ovulation for the egg to enter the uterus. Most fertilization occurs during the several days when the egg is in the Fallopian tube. If the egg is fertilzed it then attaches itself to the wall of the uterus. If fertilization has not occurred, the wall of the uterus, which has been prepared to receive the fertilized egg, begins to shed - it is this process that is referred to a the 'period'. So, as you can see, pregnancy does not occur at any time, nor does it occur during the last days of the shedding of the uterine lining.