title: Changes in the eruption order of the first permanent tooth and their relation to season of birth in Japan.
abstract: 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.
authors.affiliation: Department of Hygiene, Teikyo University School of Medicine, Tokyo, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Nonaka
authors.forename: A
authors.initials: A
authors.lastname: Ichiki
authors.forename: T
authors.initials: T
authors.lastname: Miura
mesh_terms: D000367:Age Factors; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007180:Incisor / Q000502:physiology*; D007564:Japan; D008297:Male; D008334:Mandible; D008437:Maxilla; D008963:Molar / Q000502:physiology*; D012621:Seasons; D012737:Sex Factors; D014078:Tooth Eruption / Q000502:physiology*