Best of luck throughout your pregnancy!
They are fairly accurate but an u/s can be off by as many as 2 weeks so if it's a question of paternity get a dna test done.
As far as I know they're pretty accurate. I didn't know when my last period was just kind of guessed it was April 15 and after they did my first ultrasound at 8w they left it at that and gave me an EDD. I'm 24w 6d now and it's still the same. So I'd say what they saw from your u/s is probably right.