The Human Body

Tooth Development

Humans are diphyodont—that is, they develop two sets of teeth during their lives. The first set of teeth are the deciduous teeth, 20 small teeth also known as baby teeth or milk teeth. Deciduous teeth start developing about two months after conception and typically begin to erupt above the gumline when a baby is six or seven months old. Occasionally a baby may be born with one or more deciduous teeth at birth, known as natal teeth. By the time a child is six years old, a second set of 32 larger teeth, called permanent teeth, start to erupt, or push out of the gums, eventually replacing the deciduous teeth.

Human tooth development occurs in stages. The hard tissue of the deciduous teeth, or the dentin, forms while the fetus is in the womb. After the child is born, tooth enamel develops in stages. Front tooth enamel, for example, is usually complete around one month after birth, while the enamel on the second molars is not completely developed until a child is about a year and a half old. When the enamel is fully developed the tooth erupts. Front teeth usually erupt when a child is from 6 to 12 months of age, second molars between 13 and 19 months old, and canines usually erupt at 19 months or older. The final stage of tooth development is root completion, a slow process that continues until the child is more than three years old.

Around the age of six, the roots of deciduous teeth slowly dissolve as the developing permanent teeth start to push them out. Deciduous teeth eventually fall out and are replaced by the erupting permanent teeth. This begins a transitional phase of tooth development that takes place over the next 15 years. As baby teeth are pushed out by permanent teeth, the entire mouth and jaw transform from their childhood shape to a more pronounced, adultlike structure. From age six to age nine, a child’s permanent incisors, canines, and first molars erupt. The bicuspids erupt from age 10 to age 12, and the second molars come in by age 13. The third molars, or wisdom teeth, usually erupt by the age of 21.

When human teeth grow to a certain size, the root essentially closes and the teeth stop growing. Closed-rooted teeth have narrow root openings that are only big enough for the periodontal ligament, blood vessels, and a nerve.

Permanent Teeth
The 20 deciduous teeth, also called baby teeth or milk teeth, begin to fall out when a child is about six years old. Deciduous teeth are gradually replaced by 32 permanent teeth illustrated here. The 8 incisors (4 on the upper jaw and 4 on the lower) have a straight, sharp edge for cutting and biting. The 4 pointed canines are specialized for tearing. The 8 bicuspids, or premolars, have grinding surfaces, as do the 12 more massive molars. The third molars, absent in some people, are called wisdom teeth.