OB Appointment

So my next OB appointment is on March 9th. I was told at my previous visit when I come for this one they will just see how I am and listen to the heartbeat on the fetal Doppler. I've had 3 ultrasounds at the fertility clinic before I graduated. I know at a regular OB they don't do ultrasounds as frequently. I'm plus size so I'm just curious if anyone had any experience with the Doppler picking up the fetal heart rate. And if they can't hear it can I request a vaginal ultrasound to check on baby? We'll be close to announcing at my next appointment and I don't want them to just be like "Oh, can't hear it this time we'll try again in a month".