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.

When should a dating ultrasound be done?

Dating ultrasounds are most accurate when they happen between 7-12 weeks, and are calculated by measuring the fetus crown-rump length. This generally predicts the expected date of birth within 5 days.

How early can you see if your pregnant on an ultrasound?

During this visit, an ultrasound is frequently done to confirm early pregnancy. But an ultrasound doesnt immediately show what women might expect. Its typically not until a woman is six weeks pregnant that any part of the fetus is visible, which allows the doctor to determine whether a pregnancy will be viable.