If your cycles are pretty regular, then you can be sure of your weeks even without an ultrasound
once the pregnancy
test gets positive. Yet, if you are not so sure the best way is to get a sonogram and confirm your pregnancy as well as the gestational age
. Apart form a sonogram, the only other way to know the age of the baby in early pregnancy is through the serum beta-HCG
levels and that is a tedious process. Take care.