Risk factor Relative risk or odds ratio[30] 95% confidence interval[30] Black ethnicity/race 2.0 1.8–2.2 Filipino ancestry[31] 1.7 1.5–2.1 High or low BMI 0.96 0.66–1.4 Large or small pregnancy weight gain 1.8 1.5–2.3 Short maternal height 1.8 1.3–2.5 History of spontaneous preterm birth 3.6 3.2–4.0 Being single/unmarried[3] 1.2 1.03–1.28 Bacterial vaginosis 2.2 1.5–3.1 Asymptomatic bacteriuria 1.1 0.8–1.5 Periodontitis 1.6 1.1–2.3 Low socio-economic status 1.9 1.7–2.2 Short cervical length 2.9 2.1–3.9 Fetal fibronectin 4.0 2.9–5.5 Chlamydia 2.2 1.0–4.8 Celiac disease 1.4[33] 1.2–1.6[33]