Hi mama! Based sa first ultrasound usually ang more accurate, lalo na if done in first trimester, that's what most OBs go with kasi the measurements are way more reliable than LMP, especially if you're not sure ng exact date ng last period mo. Pero honestly, both can shift pa rin depending sa baby's growth, so don't stress too much if may slight difference between the two, ask your OB which one siya gumagamit para sa baby mo, kasi siya ang makakaintindi best ng situation mo!