Table 3 The association of 40-year-old early-onset breast cancer with sociodemographic characteristics and correlated with menarcheal age

BMI (kg/m²)

  BMI < 24

2.52 (1.64–3.88)

 < 0.001

1.49 (1.24–1.78)

 < 0.001

1.76 (1.09–2.84)

0.020

1.26 (1.03–1.55)

0.028

  BMI ≥ 24

1

1.41 (1.09–1.83)

0.008

1

1.23 (0.92–1.63)

0.162

Education

  < college

1

1.20 (0.96–1.50)

0.110

1

1.15 (0.89–1.47)

0.282

  ≥ college

4.35 (2.92–6.50)

 < 0.001

1.36 (1.12–1.65)

0.002

2.54 (1.62–3.98)

 < 0.001

1.30 (1.04–1.62)

0.022

Marriage

  Unmarried/Divorce/Widowed

2.48 (1.66–3.72)

 < 0.001

1.28 (0.98–1.67)

0.066

0.88 (0.44–1.77)

0.726

1.05 (0.78–1.40)

0.749

  Married

1

1.49 (1.25–1.77)

 < 0.001

1

1.32 (1.08–1.61)

0.008

Physical activity

  None

1.50 (1.03–2.18)

0.036

1.45 (1.18–1.78)

 < 0.001

1.40 (0.90–2.17)

0.138

1.27 (0.99–1.62)

0.058

  Yes

1

1.37 (1.13–1.67)

0.001

1

1.20 (0.96–1.51)

0.113

Menopausal status

  Premenopause

7.54 (5.02–11.32)

 < 0.001

1.09 (0.87–1.37)

0.436

4.59 (2.94–7.16)

 < 0.001

1.12 (0.86–1.46)

0.397

  Peri-/Postmenopause

1

1.39 (1.13–1.71)

0.002

1

1.35 (1.09–1.69)

0.007

BMI < 24

  Premenopause

9.25 (5.63–15.18)

 < 0.001

1.20 (0.91–1.59)

0.205

6.47 (3.76–11.13)

 < 0.001

1.12 (0.83–1.51)

0.446

  Peri-/Postmenopause

1

1.45 (1.09–1.92)

0.010

1

1.42 (1.05–1.91)

0.022

BMI ≥ 24

  Premenopause

3.51 (1.59–7.75)

0.002

1.22 (0.74–2.02)

0.435

2.18 (0.93–5.12)

0.073

1.28 (0.67–2.42)

0.457

  Peri-/Postmenopause

1

1.34 (0.98–1.84)

0.067

1

1.27 (0.91–1.78)

0.161

Parity

  Nulliparous

4.71 (3.09–7.18)

 < 0.001

1.18 (0.91–1.53)

0.220

3.10 (1.53–6.29)

0.002

1.01 (0.75–1.36)

0.964

  Parous

1

1.52 (1.27–1.81)

 < 0.001

1

1.33 (1.09–1.62)

0.005

Number of births

  One

2.48 (1.42–4.33)

0.001

1.73 (1.14–2.63)

0.011

1.60 (0.85–3.01)

0.149

1.59 (0.99–2.53)

0.054

  More than one

1

1.46 (1.20–1.78)

 < 0.001

1

1.28 (1.02–1.60)

0.033

Age at first birth

  < 27 years

1

1.33 (0.99–1.79)

0.060

1

1.19 (0.85–1.67)

0.318

  ≥ 27 years

2.83 (1.69–4.72)

 < 0.001

1.52 (1.21–1.91)

 < 0.001

1.28 (0.70–2.33)

0.421

1.35 (1.04–1.73)

0.022

Lactation history

  Never

1

1.29 (0.91–1.83)

0.148

1

1.21 (0.83–1.76)

0.333

  Ever

2.21 (1.31–3.74)

0.003

1.62 (1.32–1.99)

 < 0.001

1.87 (1.06–3.28)

0.030

1.38 (1.09–1.76)

0.008

Fried food

  1 ~ 2 times a week

1

1.32 (1.14–1.53)

 < 0.001

1

1.19 (1.00–1.41)

0.055

  3 ~ 6 times a week

3.94 (2.08–7.47)

 < 0.001

1.72 (1.10–2.70)

0.019

1.97 (0.91–4.26)

0.085

1.20 (0.47–3.04)

0.707

Benign breast disease

  Never

1

1.37 (1.17–1.61)

 < 0.001

1

1.17 (0.96–1.42)

0.117

  Ever

1.14 (0.74–1.76)

0.559

1.53 (1.16–2.03)

0.003

1.11 (0.68–1.83)

0.670

1.44 (1.03–2.00)

0.031

Family history of BC

  None

1

1.36 (1.16–1.60)

 < 0.001

1

1.20 (0.99–1.46)

0.062

  Yes

1.48 (0.96–2.29)

0.074

1.56 (1.15–2.11)

0.004

1.43 (0.87–2.36)

0.163

1.32 (0.92–1.91)

0.133