Lmp was feb 18, i have shorter cycles of 25/26 days. 3 first trimester ultrasounds gave me a due date of nov 19-21 putting conception at feb 27-29. how accurate are these conception dates? anyway conception occurred before february 20?

Fairly accurate : 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). Early ultrasound due dates have a margin of error of roughly 1.2 weeks. After 20 weeks, your Dr will measure your fundal height to see if the baby's growth is keeping on track with the EDD.