Am I having a boy?

I went to the perinatologist today for my first trimester NT scan.  My husband couldn't get off from work to be there for the appointment, so when the doctor asked me if I wanted to know the gender, I said, "Yes, but write it down and put it in an envelope so my husband and I can find out together."  And then the doctor said, "Well, we'll wait until your 20 week scan so you can find out together."  I know that you can tell if it is a boy early on, and don't know it is definitely a girl, usually until around 18 weeks.  So now I feel like he knew was a boy but didn't want to say anything.  What do you think?