Obgyn only doing an ultrasound at 20 weeks

Hello ladies. Just wondering if anyone else’s obgyn only does a 20 week ultrasound and that’s it? They strictly go off of a 28 day cycle to determine a due date and I have a very irregular cycle and know that my due date is much later then what they are saying. I also had a partial molar pregnancy back in 2016 and when I fell pregnant with my son after that I got an ultrasound right away because I’m more at risk of having a repeat so I don’t understand why I’m not having the same thing done this time around. I do have my first prenatal appointment tomorrow with a nurse practitioner.