Ultrasounds/specialist question

I am 11 weeks and 5 days pregnant with di/di twins. I had an ultrasound at the fertility doctor at 6 weeks and 3 days and 7 weeks and 5 days before they released me to my regular doctor. My doctor said I don’t need another ultrasound until 20weeks,is this normal? Also, how do you know if you should go to a high risk specialist? Can you stay with your regular OB with twins if there are no issues? We did hear both heartbeats at my appointment today!spe