Table 1.

Comparison of baseline characteristics in case group and control group^a

Characteristics	Case	Control	P value
Girls
n (%)	681 (34.55)	1290 (65.45)	–
Age, years	7.02 (6.21, 7.84)	7.06 (6.30, 7.91)	0.5
Fetus age, weeks
≤ 37	13.72	12.84	0.3
38–40	69.81	73.62
≥ 41	15.56	13.69
Birth weight, kg	3.22 (2.90, 3.52)	2.61 (2.23, 3.46)	0.03
Breastfeeding for 6 months (%)	52.42	68.68	< 0.0001
High household income level b (%)	24.67	23.64	0.6
High maternal education level c (%)	25.70	22.40	0.1
High paternal education level c (%)	26.28	25.66	0.8
Maternal overweight d (%)	45.37	38.14	0.002
Paternal overweight d (%)	47.28	40.70	0.005
Maternal menarche age	12.31 (11.03, 14.17)	12.82 (11.53, 14.42)	0.06
Overweight e (%)	16.89	9.07	< 0.0001
Obesity e (%)	12.28	3.49	< 0.0001
Boys
n (%)	165 (31.43)	360 (68.57)	–
Age, years	8.22 (7.54, 8.83)	8.23 (7.67, 8.90)	0.6
Fetus age, weeks
≤ 37	13.89	12.37	0.4
38–40	69.74	73.10
≥ 41	16.37	14.53
Birth weight, kg	3.42 (2.89, 3.98)	3.37 (2.75, 3.81)	0.1
Breastfeeding for 6 months (%)	58.18	68.89	0.02
High household income level b (%)	24.24	26.94	0.5
High maternal education level c (%)	18.79	24.17	0.2
High paternal education level c (%)	24.85	25.28	0.9
Maternal overweight d (%)	41.82	37.22	0.3
Paternal overweight d (%)	43.64	41.39	0.6
Maternal menarche age	12.52 (11.01, 14.24)	12.73 (11.14, 14.45)	0.1
Overweight e (%)	20.61	14.44	0.03
Obesity e (%)	14.18	8.34	0.02

^a Values are median (25th percentile, 75th percentile) or frequencies. Test for difference between boys and girls was performed by using Wilcoxon rank-sum for non-normally distributed continuous variables and chi-square test for categorical variables

^b Average family income more than 35,000 RMB every year;

^c At least 12 years of school education

^d BMI (in kg/m²) ≥24 [21]

^e Calculated according to the Working Group on Overweight and Obesity in China criteria [20]