I have a few questions! Any advice helps

Marissa
1. My midwife is really wanting us to do an ultrasound at 20 weeks (I'm 17w3d) and since the beginning we haven't wanted any! She brought up placenta previa.. Does anyone have experience with this? From the research I have done I understand that if they do find that you have it often times it could go away by the time you're ready to deliver. 
2. Also on that topic, I understand that if they find you do have placenta previa they monitor it closely. Does that mean more ultrasounds? It kind of defeats the purpose for me if its something that usually goes away as the uterus grows. 
3. Does anyone know the effects an ultrasound has on a baby at 20 weeks?