Is it possible for an early ultrasound dating to be off by 3+ weeks in determining the 1st day of your lmp?

I suppose... how early is early? Early dating is based more on what you give as the date of your LMP more than a tiny blip on a screen.

I guess what I'm really wondering is how accurate is the 1st trimester ultrasound dating in determining gestational age. Also, can you confirm that gestational age is figured from the 1st day of lmp? If that is not known, then the u/s dating could pretty much nail it down?

I suppose... how early is early? Early dating is based more on what you give as the date of your LMP more than a tiny blip on a screen.

I'm trying to determine if my lmp was January 1st or if bleeding I had on the 26th of January was, in fact, a menstural cycle. My ultra sound dating on April 5th determined I was 10 weeks 2 days. Just want to confirm that I did get pregnant after bleeding occurred at the end of January. I'm hoping someone can confirm that if I'm 10 weeks 2 days, there would have been no way I became pregnant in January. Can you help?

1/1 - 4/5 = 94 days, 13 weeks 3 days
1/26 - 4/5 = 69 days, 9 weeks 6 days

I'm not sure what you mean by ' if I'm 10 weeks 2 days, there would have been no way I became pregnant in January?'
10 weeks 2 days = 72 days, or 1/23

We don't know if the person reading the ultrasound actually studied it or if they just rushed, making sure of heartbeat, and went by LMP. Never assume. When is your next one? Try not to puzzle too much until then, and when you see the OB, ask again.

The way this works is that your pregnancy did not officially start on the date you conceived, but rather on the first day after your last period. So, day one of the cycle during which you become pregnant is the beginning of your pregnancy.

Most women ovluate somewhere around the 14th day of their cycle, so if you are, for example, 10 weeks pregnant, you probably conceived about 8 weeks ago.

I really don't think it's likely at all that an early ultrasound would be as much as three or more weeks off. A few days plus or minus, sure but not three weeks.