Do ultrasounds measure from gestational age or does it measure from the day of conception? i have an extremely irregular period..

Ultrasound: measurements estimate the gestational age of the fetus. Gestational age is in relation to your last menstrual period. For example, a fetus measuring 8 weeks gestation on ultrasound would have been conceived 6 weeks prior to the ultrasound.A patient with 28 day cycles would have started her last menstrual period 8 weeks prior to the scan. Keep in mind that these measurements are estimates.