Abstract
Objective
The purposes of this study were to analyse trends in primary tooth emergence patterns and to identify physical factors potentially associated with them.
Methods
The participants were 27,454 infants who underwent routine 18-month-old health examinations in Ebetsu City, Japan, between 1980 and 2012. This study was conducted using data from infants’ 18-month-old health examinations over a 33-year period. The mean number of emerged primary teeth was analysed by sex using a general linear model. For logistic regression analysis, the proportion of infants with 16 emerged teeth or more at 18 months old was used as a dependent variable. Examination year; birth order; birth weight; weight, height, and chest girth at 18 months old; number of fused teeth; and mother's age were used as independent variables.
Results
The mean number of emerged primary teeth decreased over the 33-year period. Birth weight and weight and height at 18 months old decreased, and the proportion of low-birth-weight (<2500 g) infants increased over the 33-year period. On general linear model analysis, the yearly change in the mean number of emerged primary teeth was −0.0188 for boys and −0.0181 for girls. Birth weight and weight and height at 18 months old were significantly associated with the presence of 16 emerged primary teeth or more, according to the logistic regression analysis.
Conclusions
The results demonstrated that, over the 33-year period examined, the mean number of emerged primary teeth decreased and birth weight and weight and height at 18 months old were associated with the pattern of tooth emergence.
Key words: Primary tooth eruption, Associated factor, Birth weight, Japanese infant
Introduction
Long-term trends observed in the physical features of infants have been observed for more than 30 years in Japan.1 For instance, birth weight has been gradually decreasing, and the proportion of infants with birth weight lower than 2500 g, defined as low birth weight (LBW), has been increasing with time.1
Eruption of primary teeth is one aspect of physical changes over time. Because primary teeth develop in the embryonic period and emerge during infancy,2 it is assumed that tooth emergence patterns might have also been modified, along with the physical changes of infants that have been documented over the past several decades.
According to scientific records covering the 1980s, the first primary tooth emerged at 7 months after birth, on average, and the mean age at which maxillary and mandibular second primary molars emerged ranged between 2.0 and 2.5 years in Japan and the US.3,4 As for the 1990s and 2000s, several studies on primary tooth emergence timing were reported for several countries, including far-east Asian countries.5, 6, 7, 8, 9, 10, 11, 12 Woodroffe et al12 observed that overall emergence patterns of primary anterior teeth were generally later than in the ones that had been previously reported for Australian children. However, there have been only a few similar reports on primary tooth emergence in Japanese children after the 1990s.
Significant deviations between the chronological age and the estimated dental age of children may reflect underlying systemic and local disturbances.8,12,13 Therefore, more recent trends in primary tooth emergence amongst Japanese children need to be examined.
In addition, it is necessary to identify the factors that might be related to the long-term children's physical changes that might potentially affect primary teeth emergence patterns. One of the possible factors associated with delayed emergence patterns of primary teeth is thought to be the secular changes in birth weight and in weight and height at 18 months of age. Soliman et al5 observed a significant correlation between the number of emerged primary teeth and weight and height from 4 months to 36 months of age in Egyptian children. The number of emerged primary teeth tended to be lower when the infants’ weight and height were lower.5 Delayed primary teeth emergence patterns might result from the weight and height of infants decreasing continuously over a long period.
The purposes of this study were to analyse the trends in primary tooth emergence patterns and to identify physical factors.
Methods
Participants were 27,454 infants (13,906 boys and 13,548 girls) who had participated in 18-month-old health examinations in Ebetsu City, Japan, between 1980 and 2012. This study was conducted using data from infants’ 18-month-old health examinations over a 33-year period. The consultation rates during the 33 years were approximately 90%, on average, reaching 97% to 99% in the more recent 15 years. The survey items included sex, birth order, and birth weight. In addition, number of primary teeth, number of fused teeth, weight, height, chest girth, and mother's age at the 18-month-old health examination were collected at each examination. A single dentist with more than 5 years of clinical experience performed the intraoral examination using a dental mirror under artificial lighting in the public health centre during the study period. The partially emerged teeth were counted as emerged teeth. Examiner calibration was performed with 20 to 30 children every 6 months. Intra-examiner reliability of emerged primary teeth evaluated by the kappa statistic was 0.95 to 1.00.
The data of all participants (N = 27,454) were analysed. There was no missing information for birth weight, height and weight at the 18-month-old health examination, or emerged and fused teeth. The longitudinal mean number of emerged primary teeth and the proportion of infants with 16 emerged teeth or more were analysed by sex using general linear model (GLM) analysis. This analysis was also performed for the subgroup of infants without LBW (birth weight <2500 g). In addition, the long-term trend of the number of emerged primary teeth, sex difference, difference in birth weight (LBW or normal birth weight [NBW: ≥2500 g]), and interaction in sex or birth weight were evaluated by the univariate analysis of variance in GLM analysis. Furthermore, Spearman correlation coefficients between the number of emerged primary teeth and other variables were calculated by sex. The variance inflation factor (VIF) was an index to evaluate multiplex collinearity between independent variables. The VIF was calculated according to previous studies.14,15 For logistic regression analysis, the presence of 16 emerged teeth or more was used as a dependent variable, and other variables with VIFs of less than 10 were used as independent variables to avoid multiplex collinearity, including all individuals’ data during the study period (N = 27,454). There were missing values in birth order (n = 839), chest girth (n = 122), and mother's age (n = 4913). Missing values were included as blanks (unknown), and all data were used, without discarding incomplete data. This analysis was performed using statistical software SPSS Ver.24 Mac in Japanese (IBM Co., Ltd.).
This study was approved by the Ethics Committee of the Japanese Society for Oral Health (approval No. 25-3). Anonymised data were obtained in an Excel file from the public health centre. The data included only variables used for this analysis. The researchers could not identify the individual participants.
Results
The number of participants varied between 410 and 1014 from 1980 through 2012 and reached its peak in 1996 (528 boys, 486 girls). The mother's mean age (at the 18-month-old health examination) increased from 29.27 (1980) to 32.38 years (2012) (data from 1981 to 1984 are missing).
Table 1 shows the physical status of infants and the proportion of LBW. The mean birth weight decreased from 3.22 kg (boys) and 3.14 kg (girls) in 1980 to 3.03 kg (boys) and 2.97 kg (girls) in 2012. The mean body weight at the 18-month-old health examination decreased from 11.32 kg (boys) and 10.56 kg (girls) in 1980 to 10.67 kg (boys) and 10.27 kg (girls) in 2012. The mean height at 18 months decreased from 83.28 cm (boys) and 81.63 cm (girls) in 1980 to 81.08 cm (boys) and 80.11 cm (girls) in 2012. The proportion of LBW in boys increased from 2.36% in 1980 to 8.87% in 2012. In girls, it decreased from 8.59% in 1980 to 3.07% in 1982 and increased to 10.27% in 2012. The mean chest girth decreased from 49.16 cm (boys) and 47.79 cm (girls) in 1980 to 47.54 cm (boys) and 46.56 cm (girls) in 2012.
Table 1.
Infants’ physical status from 1980 to 2012.
| Year | Mean birth weight (kg) |
Proportion of LBW (%) |
Mean weight at 18 mo (kg) |
Mean height at 18 mo (cm) |
Mean chest girth at 18 mo (cm) |
|||||
|---|---|---|---|---|---|---|---|---|---|---|
| Boys | Girls | Boys | Girls | Boys | Girls | Boys | Girls | Boys | Girls | |
| 1980 | 3.22 | 3.14 | 2.36 | 8.59 | 11.32 | 10.56 | 83.28 | 81.63 | 49.16 | 47.79 |
| 1981 | 3.21 | 3.15 | 4.37 | 5.67 | 11.17 | 10.67 | 82.58 | 81.61 | 48.63 | 47.66 |
| 1982 | 3.20 | 3.16 | 3.34 | 3.07 | 11.19 | 10.69 | 82.17 | 81.03 | 48.22 | 47.21 |
| 1983 | 3.21 | 3.16 | 5.69 | 5.17 | 11.22 | 10.71 | 81.95 | 80.85 | 47.77 | 46.61 |
| 1984 | 3.22 | 3.15 | 5.25 | 7.17 | 11.27 | 10.67 | 82.51 | 81.11 | 47.52 | 46.28 |
| 1985 | 3.24 | 3.15 | 4.48 | 3.80 | 11.19 | 10.64 | 82.28 | 81.02 | 47.27 | 46.25 |
| 1986 | 3.22 | 3.14 | 4.74 | 5.15 | 11.19 | 10.58 | 81.95 | 80.74 | 47.64 | 46.24 |
| 1987 | 3.20 | 3.13 | 4.34 | 5.21 | 11.20 | 10.67 | 82.17 | 80.97 | 47.82 | 46.58 |
| 1988 | 3.22 | 3.14 | 4.24 | 4.65 | 11.25 | 10.60 | 81.92 | 80.82 | 48.02 | 46.58 |
| 1989 | 3.20 | 3.11 | 4.6 | 5.71 | 11.16 | 10.67 | 82.09 | 80.90 | 47.88 | 46.91 |
| 1990 | 3.18 | 3.09 | 4.31 | 5.99 | 11.23 | 10.62 | 82.13 | 80.67 | 48.01 | 46.90 |
| 1991 | 3.15 | 3.12 | 5.49 | 6.91 | 11.24 | 10.59 | 82.39 | 81.41 | 48.26 | 47.04 |
| 1992 | 3.13 | 3.07 | 6.05 | 6.08 | 10.99 | 10.48 | 82.24 | 81.07 | 48.05 | 46.86 |
| 1993 | 3.17 | 3.04 | 4.42 | 7.89 | 11.01 | 10.58 | 82.32 | 81.20 | 47.84 | 46.79 |
| 1994 | 3.12 | 3.04 | 7.25 | 8.81 | 10.91 | 10.39 | 82.07 | 81.09 | 48.06 | 47.07 |
| 1995 | 3.13 | 3.03 | 5.03 | 7.02 | 11.12 | 10.41 | 82.69 | 81.18 | 47.78 | 46.60 |
| 1996 | 3.10 | 3.01 | 7.58 | 10.08 | 10.95 | 10.44 | 81.88 | 80.86 | 47.64 | 46.69 |
| 1997 | 3.09 | 3.04 | 5.53 | 8.43 | 10.93 | 10.49 | 82.17 | 80.89 | 47.79 | 46.63 |
| 1998 | 3.13 | 3.01 | 4.43 | 7.82 | 10.87 | 10.24 | 81.86 | 80.68 | 47.73 | 46.55 |
| 1999 | 3.07 | 3.00 | 7.01 | 8.64 | 10.96 | 10.36 | 81.82 | 80.49 | 47.89 | 46.78 |
| 2000 | 3.10 | 3.00 | 6.86 | 9.98 | 11.01 | 10.44 | 82.15 | 80.80 | 47.69 | 46.60 |
| 2001 | 3.06 | 2.98 | 10.63 | 9.35 | 10.84 | 10.40 | 81.98 | 81.01 | 47.55 | 46.36 |
| 2002 | 3.14 | 3.01 | 5.68 | 7.13 | 10.91 | 10.31 | 81.98 | 80.57 | 47.65 | 46.47 |
| 2003 | 3.09 | 2.98 | 5.97 | 9.59 | 10.97 | 10.20 | 81.78 | 80.04 | 47.77 | 46.35 |
| 2004 | 3.09 | 3.00 | 5.29 | 10.19 | 10.78 | 10.27 | 81.48 | 80.46 | 47.49 | 46.40 |
| 2005 | 3.10 | 3.00 | 7.85 | 10.13 | 10.82 | 10.28 | 81.48 | 80.13 | 47.68 | 46.49 |
| 2006 | 3.06 | 2.98 | 9.34 | 11.47 | 10.91 | 10.22 | 81.34 | 80.18 | 47.53 | 46.40 |
| 2007 | 3.08 | 3.05 | 6.89 | 6.59 | 10.94 | 10.41 | 81.34 | 80.41 | 47.45 | 46.47 |
| 2008 | 3.09 | 2.97 | 6.96 | 11.02 | 10.84 | 10.19 | 81.48 | 80.08 | 47.64 | 46.26 |
| 2009 | 3.06 | 2.96 | 8.13 | 9.72 | 10.73 | 10.22 | 81.10 | 80.08 | 47.43 | 46.40 |
| 2010 | 3.07 | 2.97 | 6.55 | 10.73 | 10.72 | 10.30 | 81.17 | 80.11 | 47.56 | 46.60 |
| 2011 | 3.07 | 2.96 | 6.80 | 10.18 | 10.69 | 10.24 | 81.21 | 80.19 | 47.41 | 46.45 |
| 2012 | 3.03 | 2.97 | 8.87 | 10.27 | 10.67 | 10.27 | 81.08 | 80.11 | 47.54 | 46.56 |
LBW, low birth weight.
The mean number of emerged primary teeth from 1980 to 2012 and the GLM analysis are shown in Figure 1. The mean number of primary teeth was more than 15.00 from 1981 to 1991 in boys and more than 14.80 from 1981 to 1993 (except for 1991) in girls. Subsequently, they started to decrease to 14.36 (95% confidence interval [CI], 14.09–14.63) for boys (2004) and 14.10 (95% CI, 13.85–14.35) for girls (2003). The slopes of the regression lines for boys and girls were similar, with the mean number always greater in boys. In addition, the decreasing trend of the regression line did not change in the analysis without LBW infants. The GLM analysis showed the decreasing trend of the number of emerged primary teeth over the 33-year period in both sexes (P < .001). The number of emerged primary teeth was significantly greater in boys than in girls (P < .001). The interaction in sex was not observed (P = .664). The number of emerged primary teeth in LBW was significantly lower than NBW in both sexes (P < .001). The interaction in birth weight was observed in boys (P = .072) and in girls (P = .013).
Fig. 1.
Distribution of the mean number of emerged primary teeth and general linear model analysis from 1980 to 2012.
Boys: n = 13,906; boys without LBW: n = 13,086; girls: n = 13,548; girls without LBW: n = 12,507; boys without LBW: y = −0.0167x + 48.139, −0.0167, P < .001, R² = 0.4548; boys: y = −0.0188x + 52.269, −0.0188, P < .001, R² = 0.5176; girls without LBW: y = −0.0166x + 47.737, −0.0166, P < .001, R² = 0.5253; girls: y = −0.0181x + 50.669, −0.0181, P < .001, R² = 0.5544.
The proportion of children with 16 or more emerged teeth from 1980 to 2012 and the GLM analysis are shown in Figure 2. It decreased gradually from 70.75% (boys) and 66.16% (girls) in 1980 to 62.80% (boys) and 60.27% (girls) in 2012. The slope was steeper for boys than for girls.
Fig. 2.
Proportion of children with 16 emerged teeth or more (%) and general linear model analysis from 1980 to 2012.
Boys: n = 13,906; boys without LBW: n = 13,086; girls: n = 13,548; girls without LBW: n = 12,507; boys without LBW: y = −0.3467x + 760.20, −0.3467, P < .001, R² = 0.4212; boys: y = −0.3849x + 835.85, −0.3849, P < .001, R² = 0.5415; girls without LBW: y = −0.1973x + 457.26, −0.1973, P = .007, R² = 0.212; girls: y = −0.1775x + 416.82, −0.1775, P = .015, R² = 0.1757.
The number of emerged primary teeth in both sexes showed positive correlations with birth weight and weight, height, and chest girth at the 18-month-old health examination and a negative correlation with the number of fused teeth (Table 2).
Table 2.
Spearman correlation coefficients between the number of emerged primary teeth and independent variables.
| Boys |
Girls |
||||
|---|---|---|---|---|---|
| Variable | Units | r | P value | r | P value |
| Birth order | Number | 0.015 | .083 | −0.007 | .443 |
| Birth weight | kg | 0.099 | <.0001 | 0.115 | <.0001 |
| Weight at 18 mo | kg | 0.142 | <.0001 | 0.171 | <.0001 |
| Height at 18 mo | cm | 0.131 | <.0001 | 0.166 | <.0001 |
| Chest girth at 18 mo | cm | 0.085 | <.0001 | 0.091 | <.0001 |
| Fused teeth | Number | −0.222 | <.0001 | −0.175 | <.0001 |
| Mothers’ age | Years | −0.005 | .622 | 0.007 | .458 |
Table 3 shows the logistic regression analysis using the proportion of 16 or more emerged teeth as a dependent variable. All other variables were used as independent variables because no variable had a VIF greater than 10. Significant associations were found for examination year (boys OR, 0.991; 95% CI, 0.986–0.996), birth order (boys OR, 1.065; 95% CI, 1.006–1.126), birth weight (boys OR, 1.325; 95% CI, 1.193–1.473; girls OR: 1.383, 95% CI, 1.244–1.539), weight (boys OR, 1.090; 95% CI, 1.028–1.155; girls OR, 1.173; 95% CI, 1.105–1.246), and height (boys OR, 1.064; 95% CI, 1.043–1.086; girls OR, 1.082; 95% CI, 1.060–1.104) at the 18-month-old health examination and the number of fused teeth (boys OR, 0.022; 95% CI, 0.014–0.035; girls OR, 0.030; 95% CI, 0.018–0.049) with 16 or more emerged teeth. The proportion of children with 16 or more emerged teeth decreased as birth weight and weight and height at the 18-month-old exam decreased in both sexes, as birth order was lower and examination year was later in boys. On the other hand, the proportion decreased as the number of fused teeth increased in both sexes.
Table 3.
Odds ratio for the proportion of children with 16 primary teeth or more by independent valuables (logistic regression analysis).
| Boys | B | SE | Wald | Odds ratio | 95% CI | P value |
|---|---|---|---|---|---|---|
| Examination year (y) Birth order (No.) |
−0.009 0.063 |
0.003 0.029 |
11.184 4.751 |
0.991 1.065 |
0.986–0.996 1.006–1.126 |
.001 .029 |
| Birth weight (kg) | 0.282 | 0.054 | 27.444 | 1.325 | 1.193–1.473 | .000 |
| Body weight at 18 mo (kg) | 0.086 | 0.030 | 8.376 | 1.090 | 1.028–1.155 | .004 |
| Body height at 18 mo (cm) | 0.062 | 0.010 | 35.372 | 1.064 | 1.043–1.086 | .000 |
| Chest girth at 18 mo (cm) | 0.011 | 0.015 | 0.532 | 1.011 | 0.981–1.042 | .466 |
| Fused teeth (No.) | −3.817 | 0.241 | 249.965 | 0.022 | 0.014–0.035 | .000 |
| Mothers’ age (y) | −0.005 | 0.005 | 0.983 | 0.995 | 0.985–1.005 | .321 |
| Girls | B | SE | Wald | Odds ratio | 95% CI | P value |
| Examination year (y) Birth order (No.) |
0.001 −0.001 |
0.002 0.027 |
0.446 0.002 |
1.001 0.999 |
0.998–1.004 0.947–1.053 |
.504 .964 |
| Birth weight (kg) | 0.325 | 0.054 | 35.752 | 1.383 | 1.244–1.539 | .000 |
| Body weight at 18 mo (kg) | 0.160 | 0.031 | 27.160 | 1.173 | 1.105–1.246 | .000 |
| Body height at 18 mo (cm) | 0.079 | 0.010 | 58.065 | 1.082 | 1.060–1.104 | .000 |
| Chest girth at 18 mo (cm) | −0.018 | 0.015 | 1.357 | 0.982 | 0.953–1.012 | .244 |
| Fusion teeth (No.) | −3.518 | 0.256 | 189.256 | 0.030 | 0.018–0.049 | .000 |
| Mothers’ age (y) | 0.004 | 0.005 | 0.764 | 1.004 | 0.995–1.014 | .382 |
Discussion
The 18-month-old health examination including a medical and dental health checkup is a public health service in Japan, and almost all resident infants attend for such checkups with their guardians. To our knowledge, this seems to be the first long-term study of a large number of participants examined by a single proficient examiner. In this study, decreasing trends in the number of primary emerged teeth and anthropometric measurements of infants were observed over a period of 33 years. The change in the pattern of primary teeth emergence might have occurred with primary tooth eruption delay relating to birth weight and weight and height at 18 months old. There are many causes for a delay in primary tooth eruption, including various local and systemic conditions and genetic disorders.13,16 It is plausible that the decreasing trend of emerged primary teeth may imply increased incidence with local and systemic conditions and genetic disorders, as same as low birth weight.
Significant deviations between chronological age and estimated dental age in children may reflect underlying systemic and local disturbances.8,12,13 For detection of systemic and local disturbances, it is clinically significant to know the recent trend of primary tooth emergence and associated factors.
The mean birth weights of boys and girls decreased yearly in Ebetsu City. These trends of birth weight decrease and the increased proportion of LBW were the same as in a domestic report from 1980 through 2010.17,18 Short gestational age, maternal smoking habit, low prepregnancy maternal body mass index (BMI), low gestational weight gain, and low socioeconomic status were reported as factors related to LBW.19, 20, 21, 22 Kato et al17 reported that the recent decrease in birth weight of Japanese infants from 1980 to 2004 could be partially explained by the decrease in gestational age, and they also noted the increasing proportion of 20- to 39 year-old women with a BMI <18.5 kg/m2 as an additional factor. Furthermore, Yoshida et al18 reported that medical interventions for delivery, such as induction of labour and caesarean deliveries, were important factors related to the increase in the proportion of LBW infants between 1980 and 2015 in Japan.
The mean number of emerged primary teeth has been decreasing consistently from 1980 through 2012 (Figure 1). Woodroffe et al12 reported that delayed primary tooth emergence occurred by comparing the previous reports with the most recent data (2010) for Australian children, which supports the findings of the present study.
Factors related to the emergence pattern of primary teeth were examined based on the number of emerging teeth (more or fewer than16 teeth). It was found that birth weight, weight and height at the 18-month-old health examination, and number of fused teeth were significantly associated with the presence of 16 or more emerged teeth in both sexes according to the logistic regression analysis. The presence of 16 or more primary teeth occurred when birth weight and weight at the 18-month-old health examination were greater, height at the 18-month-old health examination was higher, and fused teeth were fewer at the 18-month-old health examination. Soliman et al5 reported positive correlations between anthropometric measurements (height and weight) and the number of emerged primary teeth in Egyptian infants whose ages ranged from 4 to 36 months. The positive correlation was observed in 18-month-old infants, as in the present study. Height and weight are strong physical features that reflect the degree of cell and tissue development of the child.5,23 Shanin et al24 reported that delayed primary tooth emergence was observed in infants at 6 months of age whose height measurements were below 50% and pointed out that growth parameters affect tooth emergence patterns during early infancy. Physical growth rates seemed to be relevant to the extent of tooth eruption.
Positive associations between birth weight and the number of primary teeth were observed for both sexes. Although the effect of the actual time lag remains unclear, the decrease in mean birth weight might have preceded the tooth emergence delay. Previous studies25,26 reported that primary tooth eruption tends to be delayed in LBW infants. In the present study, the number of emerged primary teeth in LBW was significantly lower than NBW in both sexes. Previous studies25,26 support the results of the present study. In addition, the decreasing trend of emerged primary teeth might promote in LBW compared with NBW. On the other hand, the long-term decreasing trends of the mean number of emerged primary teeth and the proportion of infants with 16 emerged primary teeth or more were observed in both sexes without LBW (Figures 1 and 2). The decrease in mean birth weight might be associated with the decreasing trends of emerged primary teeth in NBW infants.
The declining trends of the proportion of infants with 16 or more emerged primary teeth and the mean number of emerged primary teeth were similar, regardless of the presence of fused teeth (results not shown). The presence of fewer than 16 teeth was significantly associated with the presence of fused teeth. This result was anticipated because the number of teeth is counted as 1 when 2 primary teeth are fused. In the present study, the proportion of infants with fused teeth was low: 3.84% for boys and 3.21% for girls. Because the presence of fused teeth affects the eruption of the permanent dentition and occlusion stability,27,28 it is important to monitor the long-term trend of the incidence of fused teeth.
The number of emerged primary teeth was slightly and significantly greater in boys than in girls over the 33-year period in the present study. Tanguay et al29 reported that the tooth emergence pattern was earlier by approximately 1 month in boys than in girls, except for the first primary molar. Shinji et al3, Folayan et al6, Choi et al8, and Hitchcock et al30 also reported that primary tooth emergence was significantly8 or slightly3,6,30 earlier in boys than in girls. These reports support the results of the present study. However, the sex difference in emergence pattern remains unclear. Hitchcock et al30 reported that the sex difference in primary tooth eruption was not significantly associated with disparities in growth and psychomotor maturity between boys and girls.
Significant relationships between the proportion of 16 or more emerged primary teeth and examination year or birth order were shown for boys, but not for girls, according to the logistic regression analysis. At the 1980s, the proportion in boys (70.75%–76.81%) was more than in girls (61.34%–67.67%). The decreasing trend of proportion was steeper in boys than in girls from 1980 to 2012. These results might reflect a significant relationship between the proportion and examination year in boys. Birth order showed a positive correlation with birth weight (P < .001) for boys (results not shown). It is possible that difference in birth order might affect birth weight and emergence of primary teeth, relating to the presence of 16 or more emerged teeth.
In the present study, the trend of primary tooth emergence was evaluated based on the number of teeth at the 18-month-old health examination, before deciduous dentition is expected to be completed. It was not possible to determine the actual timing of individual tooth eruption, the completion period of deciduous dentition, and gestational ages. Therefore, delayed emergence of primary teeth and associated factors were not confirmed precisely in this study.
A large sample of Japanese infants was enrolled in the present study. However, data were obtained from a single health centre in Japan, which limits representativeness.
In conclusion, changes in the number of primary teeth, as well as factors associated with primary tooth emergence, were evaluated over a 33-year period in 27,454 18-month-old infants attending dental and medical checkups in Ebetsu City, Hokkaido, Japan. It was found that primary teeth emergence tended to be decreased, and birth weight and height and weight at the 18-month-old health examination appeared to be associated factors over the 33-year period.
Conflict of interest
None disclosed.
Acknowledgements
The authors would like to acknowledge the late Dr Keiko Kurita for providing tremendous support in the writing of this paper.
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