In my opinion, the 20-week ultrasound is a good idea for all pregnancies, but that's probably the only one that is really needed, barring other medical indications.
I only had the 20week ultrasound. I knew my dates so i didn't need a dating scan, i am low risk for downs syndrome and we are against termination (even if bub had something wrong with it) and so had no need for the 12 week one. I really wanted to see bub more often but there was no need for it.

I would be more concerned about your OB bullying you than whether your not the 20-week ultrasound is worthwhile doing.
agreed. if it's low risk pregnancy then obstetricians don't really need to see what position bub is in cuz it can change every day....so on the day of labour it will be different from every other day...why do they need to 'check' on it??