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.