You can make an approximate calculation of how long you have been pregnant by counting from the first day of your last menstrual period (LMP) to the present. This is only accurate if you have regular periods.
The most accurate way of dating the pregnancy is by doing an ultrasound. If you do an ultrasound, the doctor can tell you exactly how long you have been pregnant.
If you intend to use abortion pills at home, it is important to know how far along the pregnancy you are. Beyond 12 weeks of pregnancy, the risk of complications from using abortion pills at home increases significantly.