(I swear I thought of this before last Thursday's similarly themed CSI episode.)
Guy who thinks girlfriend's pregnancy is immaculate conception because they haven't had sex and he is sure that she has not cheated on him. Boyfriend is unreligious but becomes religious because of this. One of them works in a hospital, but not as a nurse or doctor, and they sneak an ultrasound. Unreliable narrator in the boyfriend. Is it a false positive? psychosomatic? cheating? turkey baister?