Fifteen weeks is long enough that a pregnancy would be visible on an ultrasound. I looked it up on a pregnancy website and the baby would be the size of an orange at 15 weeks, which is 4 months and in the middle of the second trimester. The only way it could be hidden is if you had a large fibroid that was blocking the view. But the results specify there are no fibroids so there's nowhere for a baby to hide.