agooboo - Male and female genitalia look exactly the same at 12 weeks. It is the "angle of dangle" that permits determination of sex at this point.
Watch this video. This is the ultrasound specialist at our hospital and apparently a bit of an authority in the field. Video is in French, but pictures speak for themselves. Watch from 3:40 onwards.
12 SA = 12 semaines d'amenorrhee = 12 weeks of "no periods", from 1st day of last period.
3:44 shows two virtually identical pictures. The one on the right is a girl fetus at 12 weeks, and the one on the left is a boy fetus of same age. (That is why I asked: "What is the "3 line sign" your sonographer was talking about?)
4:00 onwards there is a short but interesting animation of how this identical part turns into female or male genitalia over the following weeks of a fetus' life. The same point becomes clitoris in a girl and tip of the penis in a boy. Fascinating!