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.

Do ultrasounds use gestational age or fetal age?

Gestational age vs fetal age Fetal age is counted from the moment of conception. Thus, it is 14 days less than the gestational age, assuming a regular menstrual cycle of 28 days. Health care providers generally measure fetal age through ultrasounds.

Are ultrasounds ever wrong?

Ultrasounds may be used to diagnose birth defects, but they are not always accurate. Its thought that a second-trimester ultrasound, often done between 16 and 20 weeks, may detect three out of four major birth defects.