sometimes- if your tension is not quite right- your top thread can show on the bottom- or bottom thread on top---if the top thread shows on the bottom you need to tighten the top tension a little bit- if your bobbin thread shows on the top you need to either loosen the top tension or tighten the bobbin tension...it's just a matter of having them even. before you start quilting on the quilt put together a small practice piece with top, batting, backing and the threads you want to use & do some (practicing) that way you can make any adjustments you may need before starting the 'real thing'
congratulations on your soon to arrive baby