They usually go by the first day of your LMP. But not as accurate as an ultrasound.
My LMP was May 24th and due date was Feb. 28th but after ultrasound, due date changed to March 7th which means I conceived on June 13th.
If you have an 8 week ultrasound they usually go by those as they are more accurate, but your lmp is accurate within days too so if they are close it's most likely the margin of error they allow for both.
This time I was given two due dates 5 days apart when I asked about it Dr explained both were technically accurate and they will confirm at your 20week ultrasound.
We choose to use the later due date till we confirmed cause welll who wants to find out you're gonna be pregnant an extra week;)
It's all estimates based on different parameters. I also prefer to go with the later due date so I'm not expecting him to come too early! I also really don't want to be induced, so that will be my fallback as well. I'm happy to go to 42 weeks!
This is my third and I still stress about the due date even though I know it really doesn't matter! My first was 6 days before my due date and my second was 6 days after the due date. If I go from FDLP I'm due March 17th, first ultrasound measured at due date of March 25, but the 20 week measured back at March 17...but they didn't "change" the due date. They tend to stick with the first ultrasound results and just told me to expect a big boy...lol!
Same here my lmp was shown my due date as late april but ultrasound measures march 11....