what was/is your due date based off of?

Ultrasound is the more accurate

My ob said was measuring one thing, so due date based on that was March 24, but used my period date which is putting it at March 17. She's using the 17th but the initial measurement apparently showed I'd be 24th. It's apparently common and normal. Ultrasounds show a lot but I guess measuring is not perfect with them. Yours is a pretty big difference though. So not sure what I would think on that one. Mine just a week varience.

The period due date is an estimate based on the "typical" ovulation date of a "typical" cycle

Not everyone ovulates on day 14 of the cycle or has 28 day cycles, which is why ultrasound is more accurate (unless you know exactly when you ovulated)

my fertile window was supposed to be February 8th-13th. So that puts my due date around mid-November for 40 weeks right?

Ultrasound. My due date changed when I went to my first appointment. Sperms can live up to 5 days inside

First day of last period

When I was pregnant, my due date was July 31st based off of last period, on every ultrasound my baby was measuring 4 days behind (8w was 7w3d.. 19w was 18w3d.. etc) which should've put 40 weeks (based on her size) around August 4th.. but she came on her own on July 23rd/24th so it's all just an estimate anyways

My last period made my due date the 14th but my ultrasound said 21st but I gave birth the same week of the 14th

Midwife estimated 21st june from my last period day, i estimated 22nd june from my ovulation, ultrasound said 22nd june, baby came 23rd june

Ultrasound if it was done around 12 weeks. With both my pregnancies the date based off my period was totally inaccurate as I have long cycles. I was dated back around two weeks at the 12 week scan with both. In my first pregnancy I gave birth exactly on the due date given to me at my scan.

I used my LMP and matched at the doctor’s exactly via ultrasound because I was tracking my cycles. But I would say ultrasound is most accurate

I would go with the date of your last period if you truly know the day/date. This happened to me and the hospital changed my dates based on the 12 week ultrasound. I was told baby was small all through pregnancy, I kept telling them their dates were wrong but they refused to change it. They did a c section at 37 weeks gestation only to be told 4 weeks after having baby that I was in fact only 35 weeks. So yeah…make sure you know you’re dates etc