An ultrasound is actually the most accurate way to date a pregnancy because all fetuses grow at a consistent rate during the first trimester and early second. In other words, if your baby measures 9 weeks 2 days when you have your ultrasound, thats how far along you are, no matter when your last period was.

Do ultrasounds show how far along you are?

One of the biggest benefits of an ultrasound is that it can estimate how far along you are in your pregnancy (gestational age) down to just a few days. Ultrasounds estimate how far along you are in your pregnancy by comparing the growth of the fetus to established growth rates for babies around the world.