due date was July 26th, but got bumped to aug. 10th

[jackiefirsttimemom]

I just had my 20 week scan and as of this past Tuesday I'm 22 weeks along, my doctor said according to the scan I'm a week behind, so I'm 21 weeks along. But then why is my due date aug 10th from July 26th? The first day of my last period was oct. 18th and I haven't had intercourse past nov. 11th. I had my first ultrasound on dec. 31st and my doctor said the dates matched right on track so why after my 20 week scan did it change? My doctor also told me that when I have my next ultrasound, next week, we'll see the reason why my due date changed. Is it because my baby could be measuring small and that ultrasounds go by measurements??

[becca2232]

That's exactly what they go by, how big the baby is, when they measure your baby and your tummy