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.

How does the ultrasound determines the due date?

An ultrasound allows your doctor to measure the crown-rump length (CRL) — the length of the fetus from one end to the other. During the first trimester, this measurement provides the most accurate estimation for the age of the baby. Your doctor may change your due date based on the ultrasound measurement.

Whats the most accurate way to calculate due date?

The most common way to calculate your due date is to start with the first day of your last menstrual period (LMP). Add 7 days, and then count backward 3 months. For example, if your last period started on March 20, you would add 7 days to get March 27. Then subtract 3 months to get a due date of December 27.