Objective To investigate the birth weight of newborns and obesity status of preschool children in Tianjin, explore the key influencing factors of childhood obesity, and provide a basis for precise obesity prevention.

Methods From May 2015 to October 2016, data on newborns born in Tianjin were collected to establish a birth cohort. The growth and development of 58 332 newborns from birth to 5 years old were tracked using the database of the Tianjin Maternal and Child Health System. According to birth weight, Z-scores were calculated; and small for gestational age (SGA), appropriate for gestational age (AGA), or large for gestational age (LGA) were assessed. Height and weight were measured and recorded at 8 observation time points: 0.5, 1, 1.5, 2, 2.5, 3, 4, and 5 years old. Body mass index (BMI) and Z-scores were calculated to assess obesity occurrence.

Results The average gestational age at birth in the cohort was (39.56 ± 1.05) weeks, with 29 898 male infants (51.25%) and 28 343 female infants (48.75%). Based on the sex, gestational age and weight at birth, the cohort was divided into 3 groups: 2 979 SGA infants (5.11%), 48 382 AGA infants (82.94%), and 6 971 LGA infants (11.95%). During the continuous follow-up from birth to 5 years old, the lowest obesity incidence was 7.35% (3 130/42 604) at 2.5 years old and 7.41% (3 258/43 948) at 3 years old. The obesity rates of LGA children at 0.5, 1, 1.5, 2, 2.5, 3, 4, and 5 years old were consistently higher than those of AGA and SGA children of the same age. In the underweight group, the obesity rate of SGA children at age 5 (4.76%) was higher than that of AGA children (3.79%). In the obesity group, the obesity rate of SGA children at age 5 (32.26%) was higher than that of AGA children (22.67%) and LGA children (26.49%). Being SGA at birth increased the risk of catch-up growth at 0 – 6 months of age (univariate analysis OR = 5.309, 95%CI: 4.756 – 5.926, P < 0.001; multivariate analysis AOR = 5.314, 95%CI: 4.760 – 5.933, P < 0.001). Being LGA at birth increased the risk of obesity in children aged 0.5 – 5 in the normal-weight and overweight groups (all P < 0.001) and increased the risk of obesity in children of underweight mothers before age 2 (including age 2) (P < 0.05), as well as the risk of obesity in children of obese mothers before age 5 (not including age 5) (P < 0.05).

Conclusion During prenatal examinations, fetal intrauterine development should be closely monitored, and birth weight should be scientifically controlled. Avoiding the delivery of SGA or LGA infants will be beneficial for the prevention and control of childhood obesity.