Objectives Healthy-weight children tend to gain weight during winter but lose weight during summer. However, overweight elementary school children have shown accelerated summertime weight gain. Whether this seasonal growth variation occurs during preschool period is of substantial interest.

Methods Data were derived from a nationwide retrospective cohort of nursery school children. Eight consecutive sets of longitudinal measurements on height and weight were obtained from 15 259 preschool children. Thereafter, growth in height, weight and body mass index (BMI) over a period of 6 months was calculated. Summertime growth was defined as that from April to October, whereas wintertime growth was defined as that from October to April of the following year. Longitudinal growth seasonality was analysed by classifying children according to their BMI status at the age of elementary school entry.

Results Accelerated summertime weight and BMI gain were observed among children with obesity. This distinctive growth seasonality was detected from around age 2. Children having this growth seasonality at approximately 2 years of age tended to be obese at the age of elementary school entry (OR: 3.7; 95% CI: 2.9 to 4.6; p<0.0001). In height gain, obese children were growing apparently faster than those in the other groups at all ages.

Conclusion Early excessive growth with distinct seasonality was observed in preschool obese children. These findings suggest that individuals involved in child healthcare should pay closer attention to early excessive growth with distinct seasonality in preschool obesity.