Hearing is the first sense to develop completely in babies.
Within minutes of birth a newborn will startle and cry at loud noises. That's because much of the early physical development occurs long before your baby is born. Research shows that the inner ear is the only sense organ to fully form before birth. Smell also develops early in your infant's life. Other studies show that by day five a baby will turn toward a pad soaked in breast milk and by day 10, a baby shows a preference for the smell of his own mother's milk.