nub question

A few weeks ago I got my nipt blood work stating that the baby was a boy. This morning I had an ultrasound at 13 weeks and did not see any nub at all. We just lost our son in January and with him I saw a nub around this time. Although it is very early I was just curious if anyone else didn't see a nub and still is having a boy or if your blood work was inaccurate.