Table 2 Multivariable Logistic Regression Analysis of Risk Factors for Preterm Birth (PTB) Subtypes in Shenzhen, China, 2003–2012

 21-35

–

Reference

–

Reference

–

Reference

  ≤ 20

−0.400

0.670 (0.597, 0.753)

0.386

1.471 (1.422, 1.522)

−0.539

0.583 (0.546, 0.623)

  ≥ 36

0.706

2.026 (1.863, 2.203)

−0.026

0.974 (0.930, 1.021)

0.799

2.224 (2.141, 2.31)

 Bachelor

–

Reference

–

Reference

–

Reference

 Less than high school

−0.791

0.454 (0.424, 0.485)

0.074

1.077 (1.040, 1.114)

−0.627

0.534 (0.516, 0.553)

 High school and college

−0.334

0.716 (0.681, 0.771)

0.026

1.027 (0.992, 1.063)

−0.252

0.777 (0.753, 0.802)

 Postgraduate

−0.071

0.931 (0.792, 1.096)

−0.153

0.858 (0.773, 0.953)

0.092

1.097 (1.013, 1.187)

 0

–

Reference

–

Reference

–

Reference

  ≥ 1

−0.528

0.590 (0.559, 0.623)

−0.236

0.790 (0.773, 0.807)

−0.114

0.892 (0.870, 0.916)

 Appropriate

–

Reference

–

Reference

–

Reference

 Inadequate

0.485

1.625 (1.504, 1.755)

1.075

2.929 (2.806, 3.057)

0.182

1.199 (1.155, 1.245)

 Intermediate

0.399

1.490 (1.377, 1.611)

0.536

1.709 (1.634, 1.788)

0.206

1.228 (1.182, 1.276)

 Appropriate plus

−1.068

0.344 (0.306, 0.386)

−1.405

0.245 (0.226, 0.266)

−0.735

0.479 (0.455, 0.504)

 No

–

Reference

–

Reference

–

Reference

 Yes

1.755

5.786 (4.617, 7.250)

0.571

1.770 (1.449, 2.162)

1.440

4.222 (3.730, 4.779)

 No

–

Reference

–

Reference

–

Reference

 Yes

1.008

2.740 (2.166, 3.467)

−0.319

0.727 (0.596, 0.886)

0.864

2.372 (2.109, 2.669)

 No

–

Reference

–

Reference

–

Reference

 Yes

0.227

1.255 (1.029, 1.530)

−0.341

0.711 (0.634, 0.797)

2.378

10.782 (10.339, 11.244)

 Female

–

Reference

–

Reference

–

Reference

 Male

0.168

1.183 (1.129, 1.239)

0.168

1.183 (1.159, 1.207)

0.082

1.085 (1.060, 1.111)

 Hermaphroditism

−8.381

0.002 (0.000, 0.002)

1.459

4.301 (2.784, 6.644)

0.238

1.269 (0.515, 3.129)