Table 1.
Risk factors associated with spontaneous preterm birth.
| Risk factor | RR for PTB <37 weeks gestation (95% CI as applicable) | Ref. |
|---|---|---|
| Previous preterm birth of a singleton gestation | 2.62 (1.99–3.44)a | [11] |
| Short interpregnancy interval (<6 months) | 1.40 (1.24–1.58) | [12] |
| Underweight pre-pregnancy BMI | 1.32 (1.10–1.57) | [13] |
| Low socio-economic status | 1.66 (1.06–2.61) | [11] |
| Non-Hispanic black race | 1.68 (1.06–2.67) | [11] |
| Congenital uterine malformation – canalization defects (e.g. uterine septum) | 2.14 (1.48–3.11) | [14] |
| Congenital uterine malformation – unification defects (e.g. unicornuate, bicornuate) | 2.97 (2.08–4.23) | [14] |
| Maternal smoking | 1.27 (1.21–1.33) | [15] |
| Cocaine abuse | 3.53 (1.65–7.56) | [16] |
| Opioid abuse | 2.86 (1.11–7.36) | [16] |
| Family history of PTB | 1.35 (1.12–1.63)b | [17,18] |
| Pregnancy-specific risk factors | ||
| Shortened mid-trimester cervical length <2.50 cm | 6.9 (4.3–11.1)c | [19] |
| Placental abruption or vaginal bleeding in the first or second trimester | 1.62 (1.22–2.17) | [11] |
| Carriage of male fetus | 1.51 (1.02–2.24)b | [20] |
RR, relative risk; PTB, preterm birth; CI, confidence interval; BMI, body mass index.
a
Risk depends on other factors that cannot be characterized in the table, such as number of prior preterm births and gestational age of previous preterm births.
b
Data presented are odds ratios (95% CI).
c
For preterm birth <35 weeks gestation.