Background
 
The growth and development of infancy are vital to health and the quality of life throughout a person's life. We aimed to investigate the factors affecting the growth of head circumference in a sample of Iranian children under 18 months.
 
Materials and Methods: This is a retrospective cohort study conducted among 274 children aged under 18 months in Zanjan, Iran. Multi-stage cluster sampling was applied to obtain a representative sample of residents. About eight representative healthcare centers, including four urban health centers and four rural health centers, as clusters, were randomly selected. We used multiple linear regression and marginal models of Generalized Equation Estimators to examine the predictors of head circumference cross-sectionally and longitudinaly, respectively.
 
Results: Of the subjects, 49.5% (n=136) were females. The mean head circumference of newborns was 34.5±1.57. The marginal model showed that there is a significant association between sex of newborns, gestational age, high risk pregnancy, height and weight of newborns with head circumference over time (p <0.005). In multiple regression models, mothers' job and educational level also showed a significant association with head circumference in addition to sex of newborns (B=0.33, 95% CI:[0.31, 0.53]), gestational age (B=1.47, 95% CI:[0.99, 1.94]), high risk pregnancy (B=0.37; 95% CI:[0.15, 0.57]), height and weight of newborns (p <0.005).
 
Conclusion
 
In our study the mean of head circumference was similar to standard value of 34.9 cm. The sex, gestational age, high risk pregnancy, height and weight of newborns showed a significant association with head circumference, both in cross-sectional (at birth), and longitudinal models (at birth to 18 months); mothers job and educational level showed a significant association with head circumference only at birth.