— depends on your insurance. I haven't had one since I was 20 weeks. and won't get another one.
— most drs give them around 36 weeks to make sure baby is measuring on track and check the amionic fluid and they give u a guesstimate of baby's weight
— Usually it's around 37 weeks to see if she's head down around those weeks :)
— If everything is going great usually you won't. You can do a private ultrasound place though!
— you can get them whenever, it varies at every office. you get them more frequently when you have a high risk pregnancy.