Table 1 Comparison of the general characteristics between the case group and the control group

Girls

 Age, y

7.84 ± 1.05

7.71 ± 1.48

 Height, cm

133.57 ± 6.91

130.87 ± 10.05

 Weight, kg

30.30 ± 6.36

28.48 ± 6.96

 BMI, kg/m²

17.03 ± 3.14

16.49 ± 2.99

Nutritional Assessment, N(%) ²

 Obesity

34 (18.38)

23 (12.43)

 Overweight

32 (17.30)

30 (16.22)

 Normal

112 (60.54)

111 (60.00)

 Mildly emaciated

2 (1.08)

10 (5.41)

 Moderately to severely emaciated

5 (2.70)

11 (5.95)

 Heavy homework burden (Yes), %

31.52

17.39

Always live with mother (Yes), %

 Yes

89.19

95.68

 No

10.81

4.32

Always live with father (Yes), %

 Yes

84.32

88.11

 No

15.68

11.89

Dietary habits, %

 Meat and vegetable balance

26.86

37.30

 More meat and less vegetable

58.86

38.92

 More vegetable and less meat

10.29

20.54

 Others

4.00

3.24

Mothers

 Mother’s age at menarche, y

13.38 ± 1.32

13.76 ± 1.28

 Age at girl’s birth, y

27.46 ± 3.95

27.87 ± 4.18

 Illness during pregnancy (Yes), %

5.98

4.89

 Illness during lactation (Yes), %

3.80

3.26

 Exposure to toxic substances during pregnancy³ (Yes), %

7.61

4.35

Mother’s education level, %

 Middle school or lower

9.19

11.41

 High school

18.38

12.50

 College or higher

72.43

76.09

Father’s

Father’s age at first spermatorrhea, y

13.08 ± 1.32

13.76 ± 1.28

Father’s education level, %

 Middle school or lower

9.19

11.41

 High school

15.68

14.13

 College or higher

75.14

74.46

Family income (Yuan/month), %

 < 3000

1.66

1.70

 3000 ~ 5000

6.08

5.11

 5000 ~ 8000

16.02

15.34

 > 8000

76.24

77.84