What numbers on ultrasound tells how many weeks pregnant you are?

There are a few different measurements that can be taken on an ultrasound to estimate how many weeks pregnant you are. These include:

* Crown-rump length (CRL): This is the measurement of the length of the baby from the top of the head to the bottom of the buttocks. The CRL is typically used to estimate gestational age in the first trimester.

* Biparietal diameter (BPD): This is the measurement of the width of the baby's head. The BPD is typically used to estimate gestational age in the second and third trimesters.

* Femur length: This is the measurement of the length of the baby's thigh bone. The femur length is typically used to estimate gestational age in the second and third trimesters.

The gestational age is then calculated based on a formula that uses the CRL, BPD, and femur length. The formula that is used may vary depending on the ultrasound machine that is used.

In addition to these measurements, the ultrasound technician may also look at other factors, such as the development of the baby's organs and the amount of amniotic fluid, to help estimate gestational age.

It is important to note that ultrasound measurements are not always 100% accurate. The gestational age that is estimated based on ultrasound may be off by a few days or even a week.