ultrasound dating

On 2/09/10 I had an ultrasound and the Dr. said that I was 4-4 1/2 weeks pregnant. I obtained the ultrasound report, which did not give a length of pregnancy. There was a yolk sac but no fetal pole. On 3/19/10 I had another ultrasound at the same er. The report from that visit said I was 10 wks 1 day pregnant with a fetal heart rate of 170 bpm. My cycle is irregular (the last period before the one I had in Jan was either at the end of Oct or the start of Nov). On Jan 1st I had a very heavy period lasting until Jan 5th. The ultrasound tech asked my lmp but not my cycle length. If I were to estimate conception based on lmp with a 28 day cycle that would have put it at around jan 15th. If based on the ultrasound, assuming they were estimating gestational age then that would put it around Jan 21st. If the ultrasound was going by fetal age then that would put it around Jan 7th. How would the date most likely be determined at the er? I know that it is from fetal measurements but my report did not contain any. Are they most likely basing this on a 40 week or 38 week pregnancy? One concern is that I drank very heavily until feb 6th, when I had a positive home test. Also.. is it possible that I could have conceived before Jan 1st?