Why would Dr. call me in at 16 weeks?

My midwife told me that I would not get my scan until 20 wks because the baby will be completely developed then and they can check for everything. I am not sure why you are getting one @ 16 weeks but my ob in FL. always did a scan every trimester to make sure the baby and pregnancy was growing and the dates were correct. this is my first time with a midwife and I am kind of mad that they are waiting so late to do my scan.