Primary teeth are not just for chewing. Each one also acts as a guide for the permanent teeth that eventually replace them. If a primary tooth is lost too early, adjacent teeth can drift or tilt into the space. The permanent tooth may get blocked from coming into the space or may erupt out of alignment. There are several reasons that primary teeth can be lost too early: They may be missing from birth, knocked out from a fall or extracted due to severe decay. In such circumstances, a space maintainer may be recommended to keep the space until the permanent tooth is ready to come in.