Development during pregnancy is usually expressed in weeks. From the first day of your last menstrual period prior to conception to the sixth day following that is considered to be "week 0"; from the 7th day is counted as week 1 and so on. For this reason, the time prior to conception is included in the week count.
Ovulation occurs about two weeks after a menstrual period. When the egg cell is fertilized during this stage and subsequently imbeds itself in the wall of the uterus, pregnancy is considered to have begun. There is no change in the size of the uterus at this time.
Continuous cell division of the fertilized egg takes place at a rapid rate, and at 8 weeks the embryo becomes a fetus.