Even before you've learned that you are pregnant, your organism began to prepare for a new life. This work does not begin at conception, but much earlier, during the the last menstrual period. It is from this period (the first day the last menstrual period) And starts the pregnancy calendar.
Your organism preparing one of the eggs to fertilize. This work he does every month, and it takes about two weeks, after which ovulation occurs (yield mature egg to the uterus).