Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age.

How does an ultrasound determine 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.

What is considered a large baby?

Babies may be called large for gestational age if they weigh more than 9 in 10 babies or 97 of 100 babies of the same gestational age. In the U.S., this means babies born at 40 weeks gestation who weigh more than 8 pounds 13 ounces (4,000 grams) or 9 pounds, 11 ounces (4,400 grams) at birth.

Is a 7 lb baby big?

Assessing a newborns weight A babys birthweight is an important indicator of health. The average weight for full-term babies (born between 37 and 41 weeks gestation) is about 7 lbs (3.2 kg). In general, small babies and very large babies are more likely to have problems.