How did they get this due date??

Question

So I'm really confused about my due date. My sister and I did some calculations and I should be a lot further along then they say. 
So they say I'm 28 weeks. But based off my last period, I should be 31weeks. How did they get that big of a difference?
I look a lot further along then 28 weeks too. 
So my LMP was February 6th. 
They said my due date is November 27th. 
When I went for an ultrasound back at 20 weeks they said I was measuring 2 weeks ahead. So how did they get that due date when it doesn't even calculate right

Answer

When was your first ultrasound? Earlier ultrasounds will give a better dating guide. If you ovulate at a different time than what it calculated based off of the last period date, an early scan would show that baby was a different age. If your first scan was at 20 weeks then I don't know. Maybe ask your doctor why you aren't matching their calculations? Once the baby gets so big it's hard to change your date because thy all start growing at different rates 😩

Answer

Was the 20 week ultrasound your first ultrasound you had. 
By your last period you would be around 11/14 or so for a due date. My last period was 2/1 and I'm due on 11/8 and I'm almost 31 weeks and your last period started after mine so that's why i am saying that.

Answer

You add 7 days to your LP and than count 9 month so your due date supposed to be November 13 tho unless you ovulated late

How did they get this due date??

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