When does a baby say its first words, Mama and Dada? When does a baby learn to stand up and walk on its own? Is it normal for an infant to develop stranger anxiety at 9 months? In this quick and easy-to-understand lesson, we review the infant developmental milestones at 1 month, 6 months, 9 months, 12 months, 2 years, 3 years, and 4 years. This lesson includes high-yield USMLE quick take-home points, high yield summaries, and practice test questions/answers on infant developmental milestones.