Char
Although I could hear the babys H/B (I could hear a fainter, faster pulse under mine) because I could hear my H/B at the same time in the area I located, all the display would pick up was my H/B over it, as the babies was too faint in comparison to how strong mine was. It will often pick up only a strong reading, whilst you can hear other noises in the background.
You can hear your H/B in many areas of your body because your blood is pumped all through it. We have a pulse by our hips, which makes things confusing at first.
....So you may be able to hear baby, but the display won't pick it up until it becomes a stronger reading. Just listen carefully to sounds in the background. Babys HB will sound like quiet "Galloping Horses" or a train - Where as yours will be much slower.