The present study found that preterm birth was associated with a 40% decreased prevalence of dental caries. Our findings are at variance with a previous cross-sectional study which showed that preterm birth was significantly positively associated with the prevalence of dental caries among US children aged 3 to 5 years . On the other hand, our results showing no associations between LBW or SGA and dental caries are in agreement with those of other studies [4–7]. In a cross-sectional study in Brazilian children aged 6 years, a lack of association between LBW and a high level of dental caries (number of decay, missing and filled teeth ≥4) was observed . According to a systematic review regarding the LBW and dental caries based on four studies in 2001, there was no evidence that LBW is a risk factor for caries in the primary dentition . On the other hand, a prospective cohort study in the UK, in which birth weight was treated as a continuous variable across the whole range, showed that birth weight was positively associated with the risk of dental caries in 985 children aged 61 months: the adjusted OR per 100 g increase was 1.08 (95% CI: 1.03–1.13) . Regarding SGA, a US cross-sectional study using data from the Third National Health and Nutritional Examination Survey showed that SGA was non-significantly inversely associated with dental caries in children aged 2 to 5 years . However, it should be noted that the above mentioned studies used different definitions of outcome, study population, assessment methods, and confounding factors, thus limiting the feasibility of inter-study comparisons.
A longitudinal study among Australian children at 52 months of age showed that the prevalence of enamel defects, such as hypoplasia or opacity, was higher in preterm children than in full-term normal birth weight children . It is speculated that these enamel defects predispose children to increased caries risk . Nevertheless, the prevalence of dental caries in these two groups in the cited study was not significantly different . Further, in a cross-sectional study among Brazilian children aged 0 to 3 years, the mean number of dental caries was higher in children born at full term than in those born prematurely . Our observed marginally inverse association between preterm birth and dental caries may be explained by the fact that preterm birth children experienced more follow-up and had a greater opportunity to receive various forms of health information, including information on oral health, from the hospital than did full term birth children. Another possible explanation is that the delayed tooth eruption in children born preterm might contribute to a decreased risk of dental caries. In a retrospective study in Turkey, compared with children at >37 weeks of gestational age, children at ≤37 weeks of gestational age demonstrated a significant delayed eruption of the first primary tooth . Alternatively, this association may be merely a coincidence.
No previous studies have addressed an interaction between birth conditions and prenatal smoking with respect to dental caries. Maternal smoking during pregnancy is the leading cause of LBW and preterm birth. Our previous study showed that prenatal smoke exposure was associated with an increased prevalence of dental caries in children . Maternal smoking during pregnancy, LBW and preterm birth, and dental caries might be interrelated. In the current study, no interaction was observed between preterm birth and maternal smoking during pregnancy with respect to dental caries, however.
This research study had several methodological strengths. Study subjects were homogeneous in terms of age and geographic background. Information on birth weight and gestational age at birth was drawn from data recorded by hospital or clinic staff in the maternal and child health handbook. Data regarding dental caries were obtained from dental examinations by dentists. We were thus able to control for a variety of potential confounding factors.
Several methodological limitations of the current study should also be clarified. Of 8,269 eligible children in Fukuoka City, only 2055 (24.9%) were included in this analysis, leading to a potential for selection bias. The study subjects were probably not a representative sample of Japanese toddlers in the general population. In fact, the educational levels of the subjects’ parents in the present study were higher than those of the general population. According to the 2000 population census of Japan, the proportions of men 35 to 39 years of age in Fukuoka City with < 13, 13 to 14, ≥15, and unknown years of education were 39.6%, 8.0%, 43.3%, and 9.1%, respectively . The corresponding figures among fathers in the present study were 27.6%, 15.0%, 57.4%, and 0.0%, respectively. The proportions of women 30 to 34 years of age in Fukuoka City with < 13, 13 to 14, ≥15, and unknown years of education were 41.3%, 34.4%, 16.1%, and 8.3%, respectively . The corresponding figures among mothers in the present study were 28.3%, 40.0%, 31.7%, and 0.0%, respectively. On the other hand, the prevalence of dental caries in the study population (20.7%) appeared lower than that in a sample of 3-year-old Japanese children assessed in a 2005 survey of dental disease . Further, it is difficult to generalize our findings to other populations because various factors, such as cultural and social factors and the prevalence of outcome, are different across populations. Although adjustments were made for several confounding factors, residual confounding effects could not be ruled out.
In this study, because the data on dental examinations were transcribed by parents or guardians of the children from their maternal and child health handbook to our self-administered questionnaire, we cannot exclude the possibility that transcription errors occurred. However, misclassification of outcome is unlikely to differ across categories of birth conditions. The non-differential outcome misclassification might have biased the magnitude of the observed associations toward the null.