How does an unborn baby know its father?

Unborn babies react to the sound of their father's voice, in the womb, and after birth will turn to their father's voice over another man's voice. There is no way for a fetus to know that a specific male voice belongs to the biological father. It is more likely that, after birth, the male voice that is heard most often by the baby and that becomes associated with pleasure (feeding, being picked up, etc.) will contribute to the initial bonding that occurs, and that will strengthen over time, between baby and father. Later, visual cues will add further strength to the original bonding.