16 hour shifts + 6 months pregnant

I’m a nurse and work 12 hour shifts & im currently 6 months pregnant. Since I’ve told my bosses that I’m pregnant, I’m always the one to get mandated to stay when someone on nightshift calls off. After 12 hours, I can feel my socks cutting into me because my feet and legs are so swollen and my back hurts awfully. When I’m mandated , I have to stay 16 hours and it’s too much for me. I hurt too bad. My pregnancy is healthy and I’ve had no problems, other than testing positive for strep B. But do you think my doctor would write me a note limiting me to 12 hour shifts?