Ay mama, ang obstetrics ultrasound talaga ang mas accurate para sa dating ng pregnancy kasi yun ang specialized para dito! Ang second one from the sonographer might have a slight difference, pero trust the OB's reading more, usually that's what they go with for your due date anyway, kaya stick with March 20 na lang para hindi ka ma-confuse. If you're still worried, just ask your OB next visit kung bakit may difference ang dates, baka may explanation sila!