What physical milestone is expected in infants by 9 months?

By 9 months of age, infants typically reach the milestone of crawling, which involves moving across the floor on their hands and knees. This developmental stage signifies increasing mobility and coordination, as babies learn to support their weight on their arms and legs and navigate their environment. Crawling also promotes muscle development and enhances the ability of infants to explore and interact with their surroundings.

While other milestones like standing unassisted and walking occur later in a child's development, waving goodbye is more of a social or communicative behavior rather than a physical milestone. Thus, crawling is appropriately recognized as the expected physical milestone for infants by 9 months.