I found out with both my kids at 16 wks. First was a boy(very obvious), and now my lil girl who is due feb 1st.
If they saw the three lines than you should be in the clear, a little boy part would be quite noticeable.
If you had 2 techs tell you girl I would feel fairy confident, especially at those gestations, but they can be wrong so don't take your tags off all the clothes. Will you get anymore ultrasounds? Good luck & congrats!