Looks like we won't be having that second ultrasound afterall. My doctor said he got all the information he needed and that the ultrasound can be done anytime between 16 and 20 weeks (which is what I thought too).
I'm a little peeved at the ultrasound tech who decided on her own that because I was 17 weeks instead of 19 I would have to come back again. It was frustrating because after she discovered the age and she did the measurements she rushed through everything else giving us one crap picture saying, "You'll be back in two weeks and you'll get more pictures." She did try and find the sex of the baby which she concluded was a girl because of the lines. I feel a little cheated out of the only ultrasound I'll have.
So whatever. But if you're not a doctor don't act like one. Just needed to let that out.