Baby’s Developments
The due date of your baby is calculated as first day of your last periods + 7 days + 9 months
So for example, if your last period started on the 21st of January, then your due date would be 21st January + 7 days + 9 months.
Accordingly, during week that is counted as the first week of pregnancy, you are not actually pregnant because as per the preceding example, your first week of pregnancy would start on the 21st of January, which is the first day of your period. So for now, the egg that will eventually become your baby is still in either one of your two ovaries. Week one begins the first trimester of your pregnancy.
Changes in the Mother
From the end of your last period, the inner lining of your uterus (which is called the endometrium) has been thickening. Since no implantation took place during that period cycle, this thickened layer will be removed from your uterus, starting from the first day of this period.