Maybe it has to do with the fact babies move around a lot and can cause pain which would cause a spike in pulse thus messing up the results. I know if I got a sharp rib kick my pulse would raise substantially for a second. Maybe the stress of taking one could be dangerous. Sorry that was such a spread out answer I just don't know but could think of a few possible answers.
Lie detectors depend on your pulse, blood pressure and similar factors.
Because there is another body inside the person being tested, it would make the results unpredictable.
The baby's heart is not beating at same pace as moms heart and blood pressure is different.
The lie-detecting machine probably would pick up the heart-beat of the baby as well, causing false spikes in the reading.
Radiation is not used in these kinds of tests
Maybe because of radiation?