Changes in the eruption order of the first permanent tooth and their relation to season of birth in Japan.

In order to examine the eruption order of the first two permanent teeth, kindergarten children 5 to 6 years old were examined in Hakone, Japan. Among a total of 817 children examined from 1976 to 1984, 349 were determined as I-type children, whose mandibular first incisor erupted earlier than the mandibular first molar, and 183 were as M-type children, whose first molar erupted earlier in the mandible. The mandibular I-type rate, i.e., the proportion of the I-type among a total of I- and M-type children, was 66% (349/532). In 1983-1984, the I-type rate was 70% among boys and 62% among girls, but the overall I-type rate did not differ significantly by the sex of the subject or by the year of examination. The I-type rate varied significantly with the season of subject's birth. Those born in October or November showed a significantly lower I-type rate (33%) than the other subjects (P less than 0.001), in spite of a shift of birth season with low I-type rate toward winter in 1983-1984. The difference of the I-type rate according to birth season suggests that the causes responsible for this change are primarily environmental and act at the prenatal or perinatal stage of life.