Understanding Ultrasound Dating in Early Pregnancy: LMP vs. Fetal Age

Question
Does ultrasound date you from estimated lmp or exact fetal age ive been getting told alot of different things And when doctor tell you how far along you are is it from lmp i had a period in october around the end of the month just dont member the date and a ultrasound puts me around the end of october but i missed my period in november. so why did  the ultrasound put me two weeks back and say i conceived in november im confused

Answer
The USG takes measurements of different parameters like crown rump length, femur length and head circumference and so on. These values are then compared with the standard values for the population (which has been acquired by doing USG of many thousand pregnant women). Based on this graph the fetal age is estimated. Therefore it is the most accurate assessment of fetal age. And don't worry, there will always be a difference of 1-2 weeks between the USG age and age as determined by USG. So if you missed you period at end of november, then the USG will show conception around mid nov. This is normal and to be expected.