Baby teeth, more accurately known as primary teeth or deciduous teeth, are the first of two sets of teeth that people develop during their lifetimes. They begin to form during the first trimester of pregnancy and have fully formed by the time an infant is born. However, primary teeth usually remain hidden under the gums until they begin to emerge (erupt) between 5 months and 8 months of age. This process is known as teething. 

A human tooth is composed of several parts:

• Crown. Part of the tooth that is visible above the gum line.
  
  
• Enamel. The hard, translucent outer shell that covers the crown. The enamel is hard enough to handle the strain of a lifetime of biting, chewing and grinding. It also protects the tooth from bacteria and changes in temperature when eating hot or cold foods. Nonetheless, it is brittle and may crack or chip.
  
  
• Root. Part of the tooth that pushes into the upper (maxilla) or lower (mandible) jawbone. Each root has a tiny opening at the bottom called the apical foramen. Blood vessels and nerves enter the root through this space.
  
  
• Cementum. Hard shell that covers the root and attaches to fibers that fasten the root to the jawbone.
  
  
• Dentin. Layer of the tooth found beneath the enamel and the cementum. It makes up most of the tooth’s structure and is responsible for the tooth’s color. It typically has a whitish to yellowish hue.
  
  
• Pulp. Located beneath the dentin at the core of the tooth. It is made up of blood vessels, nerves and connective tissue. The blood supply of the pulp provides the nutrients that keep the tooth alive. The pulp is made up of two parts. The pulp chamber is found in the crown of the tooth. The root canal is the part of the pulp located in the root of the tooth, where blood vessels and nerves enter the tooth.
  

  

Twenty primary teeth – eight incisors (four front teeth on both the upper and lower jaw), four canines (located in the front of the jaw behind the incisors) and eight molars (larger teeth in the back of the mouth) – typically erupt by the time a child is 3 years old. Eruption of the lower teeth generally precedes eruption of the top teeth. Eruption times for girls are generally earlier than those for boys.

The following schedule for the top primary teeth was established by the American Dental Association (ADA):

• Central incisor: 8 months to 12 months
• Lateral incisor: 9 months to 13 months
• Canine (cuspid): 16 months to 22 months
• First molar: 13 months to 19 months
• Second molar: 25 months to 33 months

Eruption of the bottom primary teeth occurs as follows:

• Central incisor: 6 months to 10 months
• Lateral incisor: 10 months to 16 months
• Canine (cuspid): 17 months to 23 months
• First molar: 14 months to 18 months
• Second molar: 21 months to 31 months

The first primary teeth to emerge are usually the bottom two incisors, followed by the top four incisors and then the rest of the bottom incisors. After the incisors the rest of the primary teeth begin to fill the mouth, usually with two developing at a time. The roots of primary teeth are 50 percent formed when eruption first begins, and roots are completed by the time a child is between the ages of 18 months and 3 years.

Primary teeth have several roles. They help with biting and chewing, and are crucial to a child’s ability to speak. They may also make a child appear more attractive. Finally, primary teeth serve as a guide for permanent teeth. For this reason, primary teeth usually have more space between them to allow permanent teeth more room to erupt.

Primary teeth are smaller and whiter than permanent teeth. Some of the other ways in which primary teeth differ from permanent teeth include:

• Crowns are shorter in primary teeth
  
  
• Enamel depth is more consistent and thinner in primary teeth
  
  
• Pulp chambers of primary teeth are larger relative to the size of the tooth
  
  
• Roots are longer and more slender in primary teeth

Eventually, a child’s primary teeth begin to fall out. The middle teeth in front (incisors) typically fall out at age 6. Molars in the back are shed between the ages of 10 and 12. By age 13, children usually have their permanent teeth.