To study the association between birth weight and dental caries in 7-years-old children.

All 498 children aged 7 were recruited from a primary school in Shanghai at 2014. Oral examinations were performed by calibrated dentists according to World Health Organization procedures and diagnostic criteria, which decayed, missing and filled teeth were recorded. Birth weight was collected through self-administrated questionnaires by children′s parents. After excluding 67 participants without reported birth weight, 431 children were recruited into analysis. Chi square test was applied to test the difference of caries prevalence in between respective groups as well as the t test and Dunnett test for the difference of decayed teeth and birth weight. The association between birth weight and severe caries was analyzed by Logistics regression.

The prevalence of dental caries was 70.1%, and the mean of decayed, missing and filled teeth (dmft+DMFT) were 3.44 with a standard deviation (SD) of 3.55. The average birth weight of participants was (3.30 ± 0.51) kg. For children with birth weight of<2500 g, 2500-2999 g, 3000-3499 g, 3500-3999 g and ≥4000 g groups, the dmft+DMFT were 4.09 ± 4.23, 2.25 ± 2.48, 3.42 ± 3.59, 3.76 ± 3.56 and 4.11 ± 4.06 respectively; and the prevalence of severe caries (dmft+DMFT≥5) were 40.9%, 15.9%, 35.0%, 39.4% and 41.7%, respectively. Among the children with birth weight ≥2500 g, children with severe caries had birth weight significantly higher than those without caries or with mild caries (mean-difference_{caries-free-severe-caries}=-0.12, P_{caries-free-severe-caries}= 0.047; mean-difference_{mild-caries-severe-caries}=-0.14, P_{mild-caries-severe-caries}= 0.009) . Compared to the children of with birth weight 2500-2999 g, the odds ratios (OR) of severe caries of groups with birth weight <2500 g, 3000-3499 g, 3500-3999 g and ≥4000 g were OR_<2500g= 3.37 (95% confidence interval 1.13~10.05) , P_<2500g=0.030; OR_3000~3499g= 2.69 (1.28-5.67) , P_{3000~ 3499 g}= 0.009; OR_{3500~ 3999 g}= 3.16 (1.46-6.84) , P_{3500~ 3999 g}= 0.004; OR_{≥4000 g}= 3.45 (1.33-8.98) , P_{≥4000 g}= 0.011.

A relationship was found between birth weight and dmft+DMFT or severe caries among children aged 7. Both Low and high birth weight were probably risk factors for severe caries.