Table 3 Univariate and multivariate analyses of factors associated with sPTB in twin pregnancies within the training set

Age (years), n (%)

 < 35

Ref.

Ref.

 ≥ 35

2.11 (1.57–2.83)

< 0.001

2.28 (1.67–3.13)

< 0.001

Pre-pregnancy BMI (kg/m²), n (%)

 Normal

Ref.

Ref.

 Underweight

2.22 (1.54–3.20)

< 0.001

2.36 (1.60–3.47)

< 0.001

 Overweight

1.67 (1.13–2.49)

0.010

1.67 (1.09–2.56)

0.018

 Obesity

7.71 (2.99–19.91)

< 0.001

10.45 (3.91–27.87)

< 0.001

Nulliparity, n (%)

 Yes

Ref.

Ref.

 No

0.72 (0.52–0.98)

0.038

0.58 (0.41–0.82)

0.002

Chorionicity, n (%)

 Dichorionic diamniotic

Ref.

–

–

 Monochorionic diamniotic

1.37 (0.98–1.91)

0.064

–

–

 Monochorionic monoamniotic

1.26 (0.11–13.96)

0.850

–

–

 Conception modalities, n (%)

–

IVF

Ref.

–

–

 Spontaneous conception

1.18 (0.91–1.53)

0.213

–

–

 Ovulation induction

1.62 (0.62–4.23)

0.322

–

–

Pre-pregnancy diabetes, n (%)

 No

Ref.

Ref.

 Yes

5.85 (3.36–10.21)

< 0.001

5.81 (3.24–10.39)

< 0.001

Gestation diabetes, n (%)

 No

Ref.

–

–

 Yes

1.07 (0.81–1.39)

0.644

–

–

Pre-pregnancy hypertension, n (%)

 No

Ref.

Ref.

 Yes

2.89 (1.55–5.42)

< 0.001

2.79 (1.44–5.41)

0.002

Hypertensive disorders of pregnancy, n (%)

 No

Ref.

–

–

 Yes

0.74 (0.51–1.09)

0.130

–

–

Scarred uterus, n (%)

 No

Ref.

–

–

 Yes

1.36 (0.87–2.11)

0.177

–

–

Infections, n (%)

–

 No

Ref.

–

 Yes

1.37 (0.92–2.04)

0.118

–

Cervical incompetence, n (%)

 No

Ref.

Ref.

 Yes

4.47 (2.75–7.25)

< 0.001

5.12 (3.08–8.48)

< 0.001

Thyroid abnormalities, n (%)

 No

Ref.

–

–

 Yes

0.95 (0.64–1.41)

0.816

–

–

Anemia, n (%)

Normal

Ref.

–

–

 Mild

1.43 (1.03-2.00)

0.693

–

–

 Moderate

1.29 (0.83–2.01)

0.248

–

–

 Severe

1.80 (0.82–3.94)

0.454

–

–

Hypoproteinemia, n (%)

 No

Ref.

–

–

 Yes

1.23 (0.82–1.83)

0.311

–

–

Thrombocytopenia, n (%)

–

 No

Ref.

–

–

 Yes

0.69 (0.41–1.19)

0.190

–

–