Table 1 Logistic regression for risks underlying preterm birth. OR (Odds Ratio), 95 % CI (95 % confidence interval), p-value (significance). Significant results are indicated in bold.

Risk factor*	OR**	95 % CI**		p-value
		lower	upper
* Reference categories are not listed, e.g. singleton birth, girl, multipara (child with older siblings), country, old federal states, rural area, low social status, Germany, under 20 years of age, 19–< 24 kg/m2.** An OR of more than 1 indicates a higher risk; it is significant when the upper and lower 95 % CI are over 1. An OR of less than 1 indicates a lower risk. It is significant when the 95 % CI is always less than 1.
Pregnancy factors
Multiple pregnancy	13.116	10.896	15.790	0.000
No information	1.012	0.526	1.944	0.972
Boys	1.117	1.010	1.234	0.030
Maternal factors
Nullipara	1.470	1.322	1.634	0.000
Maternal age				0.023
20–29	0.983	0.750	1.287	0.898
30–39	1.182	0.891	1.567	0.247
40 and above	1.212	0.745	1.971	0.440
No information	0.775	0.323	1.860	0.568
Maternal height (per10 cm)	0.804	0.746	0.867	0.000
Maternal BMI				0.002
< 19 kg/m2	1.315	1.057	1.637	0.014
24–< 26 kg/m2	0.879	0.759	1.018	0.085
26–< 28 kg/m2	0.952	0.798	1.135	0.581
28–< 30 kg/m2	0.764	0.613	0.951	0.016
30–< 35 kg/m2	0.799	0.654	0.977	0.029
≥ 35 kg/m2	1.109	0.865	1.421	0.415
No information	0.742	0.401	1.370	0.340
Social factors
Social status				0.000
middle	0.805	0.713	0.910	0.001
high	0.701	0.606	0.812	0.000
New federal states	1.168	1.021	1.335	0.024
Urban area				0.008
small town	1.239	1.056	1.453	0.009
midsize town	1.315	1.123	1.541	0.001
large town or city	1.202	1.019	1.419	0.029
National origins of mother				0.011
Europe/North America/Australia	1.112	0.943	1.310	0.206
Turkey/Near East/Middle East/North Africa	1.501	1.207	1.867	0.000
Sub-Saharan Africa	1.292	0.510	3.272	0.589
Latin America	1.262	0.584	2.726	0.554
Asia	1.479	0.996	2.197	0.053
No information	1.006	0.580	1.746	0.982
Constant	3.966			0.030