A lthough a human baby is born without teeth, a complete set of 20 deciduous , or baby, teeth (also called primary teeth ) already has formed within the gums of the offspring while it still is within the mother's womb. The buds of the permanent or secondary teeth are developing even before the first baby tooth appears at around the age of six months. The baby teeth obviously are formed from foods eaten by the mother. Generally, if the mother follows a good diet during pregnancy, no special food items are required to ensure an adequate set of deciduous teeth in the baby.

It takes about two years for the full set of deciduous teeth to appear in the baby's mouth. The first, usually a central incisor at the front of the lower jaw, may erupt any time between the ages of three and nine months. The last probably will be a second molar at the back of the upper jaw. As with walking, talking, and other characteristics of infants, there is no set timetable for the eruption of baby teeth. One child may get his first tooth at three months while another must wait until nine months, but both would be considered within a normal range of tooth development.

The permanent teeth are never far behind the deciduous set. The first permanent tooth usually appears around the age of six years, about four years after the last of the baby teeth has erupted. As the baby teeth gradually fall out, they are replaced by permanent teeth. The chart below shows the usual ages for the appearance and shedding of baby teeth.

UPPER JAW
central incisor	8-12 mos	6-7 yrs
lateral incisor	9-13 mos	7-8 yrs
canine (cuspid)	16-22 mos	10-12 yrs
first molar	13-19 mos	9-11 yrs
second molar	25-33 mos	10-12 yrs

LOWER JAW
central incisor	6-10 mos	6-7 yrs
lateral incisor	10-16 mos	7-8 yrs
canine (cuspid)	17-23 mos	9-12 yrs
first molar	14-18 mos	9-11 yrs
second molar	23-31 mos	10-12 yrs

Tooth Decay

I n addition to wear, tear, and injury, the major threat to the health of a tooth is tooth decay, or caries . Tooth decay and gum diseases are the leading causes of tooth loss. Tooth decay is caused by the bacteria that are normally present in the mouth and in the foods we eat. The bacteria digest the sugars and starches in the particles of food that remain in the mouth and begin to produce harmful acids within 20 minutes after eating. Although saliva and the actions of the tongue generally wash away some of the harmful material, decay will occur in places where bacteria and food particles accumulate and remain undisturbed.

The bacteria and acids build up in the mouth and become part of plaque , a sticky, transparent substance that forms a film over the surface of the teeth. Plaque forms on a continuous basis, which is the reason teeth must be flossed and brushed daily. Plaque can grow between the teeth and gums and irritate the soft tissues that support the teeth. The acids in plaque can eat through tooth enamel, creating a cavity. Plaque that is not removed combines with minerals in the saliva and hardens into a rough-textured substance called tartar , or dental calculus . Tartar can only be removed with a professional cleaning.

Other Causes of Decay

Bacterial acid is not the only way in which the tooth enamel may be damaged to permit the entry of decay bacteria. Certain high-acid foods and improper dental care can erode the molecules of enamel. Temperature extremes also can produce cracks and other damage to the enamel; some dental scientists have suggested that repeated exposure to rapid temperature fluctuations of 50° F, as in eating alternately hot and cold foods or beverages, can cause the enamel to develop cracks.

Complications of Tooth Decay

Tooth decay occurs gradually. It begins on the tooth's outer enamel surface where plaque has formed. The initial stage of tooth decay is usually painless and often goes unnoticed. Once decay activity breaks through the hard enamel surface, the bacteria can attack the dentin. Because the dentin is about 30 percent organic material, compared to 5 percent in the enamel layer, the decay process can advance more rapidly there. If the tooth decay is not stopped at the dentin layer, the disease organisms can enter the pulp chamber, which contains sensitive nerve endings. The decay can produce an acute inflammation, or abscess, which, if unchecked, can spread to adjoining teeth or other parts of the body. Osteomyelitis, an infection of the bone and bone marrow, and endocarditis, an inflammation of the lining of the heart, are among diseases in other parts of the body that can begin with untreated tooth decay.

Periodontal disease, described below, is another possible complication of tooth decay.