Accurate ultrasound?

I had a ultrasound at 10weeks and 1 day I am due in March and was wondering how accurate is the first ultrasound? Is it more likely to be further along than expected? How long does it take from conception for the baby to be in the uterus and starting on week one?

Ultrasound in the first trimester is the most accurate time to date a pregnancy. In the first trimester, the baby is measured from head to rump (crown rump length), and this is extremely accurate.....

"Today, the crown rump length is a universally recognized term, very useful for measuring early pregnancies. The CRL is highly reproducible and is the single most accurate measure of gestational age. After 12 weeks, the accuracy of CRL in predicting gestational age diminishes and is replaced by measurement of the fetal biparietal diameter."

http://www.obgyn.ufl.edu/obgyn101/Ultras...

"Early in pregnancy (1st trimester), fetuses of the same gestational age all are nearly identical in size. At this time in pregnancy, there is very little difference in size between those fetuses who ultrimately will be large, average size, or small. As pregnancy continues, these genetic and constitutional differences in size become more important clinically. It is not possible, for example to separate a 20 week fetus who is large from a 21 week fetus of normal size, or a 22 week fetus who is small for its age. All three will measure as though they were 21 weeks......The accuracy of ultrasound in predicting gestational age gets worse as the pregnancy advances. By 20 weeks, ultrasound is accurate only to within plus or minus two weeks, and by the third trimester, its accuracy falls to plus or minus 3 weeks."

http://www.obgyn.ufl.edu/obgyn101/Ultras...

I have the link to a site that may help you find even more information (and doctors there that you can ask this of). It's at: http://forums.obgyn.net/pregnancy-birth