Evidence suggests that ultrasounds more accurately predict your due date than using your last menstrual period—but only in the first trimester and early second trimester (until roughly 20 weeks). 3 Early ultrasound due dates have a margin of error of roughly 1.2 weeks.

Can ultrasound be wrong about pregnancy?

The most common reasons for a pregnancy not appearing on the ultrasound scan are: it is too soon to see the baby on the scan. you have had a miscarriage. the pregnancy is outside the womb (an ectopic pregnancy)

Why does pregnancy start from your last period?

If you were having regular periods before pregnancy, your doctor will calculate your due date based off of your last menstrual period. This goes back to the fact that in order to get pregnant, your body ovulated—or released an egg—roughly in the middle of your cycle and it was fertilized by sperm.

How accurate is a 20 week gender ultrasound?

With that being said, over 99% of ultrasounds performed between weeks 18 and 20 will make the correct determination. 2 It is only when it is performed before week 14 that the rate of accuracy can drop significantly.