Honestly I don't care what shots they get, you can't trust it 100% unless you got an amnio or other test that looks at the actual genetic makeup. There are plenty of stories out there of women being told several times they are having one or the other and get the opposite.
We aren't finding out the sex of our baby. What is between his or her legs makes no difference in how we will prepare for a new baby's arrival. I hate the idea that you need to know to "prepare" as if we are being reckless or are unprepared. There is literally nothing I can think of you need for a boy and not a girl, or the other way around. Not getting on to you, this just irritates me.
If you really want to know, go pay for a 3d scan in a few weeks.
Re: 20 week Ultrasound