They can be pretty accurate, but it depends a lot on when you get your first ultrasound...the further along you are, the less accurate they are. Ultrasounds in early pregnancy are usually considered pretty accurate (within about a week) because the baby in early stages develops in a very particular pattern. How far along are you?
Typically yes they are fairly accurate give or take a day or so, if you get and early ultrasound. Ultrasounds done later on could be off depending what rate your baby is growing
I had my first ultrasound at 9w6d. then i had an ultrasound at 13w 6d aand was measuring 14w4d.
i found out with my first when i was about 4 months pregnant with my first and he was born 3 weeks before the due date weighing 6lb 14oz so i think that it was pretty accurate you can prob say give or take a week with the later dating u/s but the ones before 12 weeks with be pretty right on within days