Table 4.
Association between birth timing and obstetric history, sociodemographic, and clinical risk factors (Cox regression model)
| Hazard ratio (95% CI) |
|||
|---|---|---|---|
| Spontaneous | Indicated | Total | |
| Sociodemographic factors | |||
| Maternal race | |||
| Caucasian | ref | ref | ref |
| Black or African American | 2.08 (0.54-8.03) | 1.94 × 109 (0.00-inf) | 1.73 (0.49-6.12) |
| Other | 1.42 (0.49-4.15) | 0.01 (0.00-0.54)a | 1.17 (0.46-2.97) |
| Paternal race | |||
| Caucasian | ref | ref | ref |
| Black or African American | 0.67 (0.19-2.37) | 5.78 × 10-10 (0.00-inf) | 0.70 (0.21-2.33) |
| Other | 0.68 (0.23-2.04) | 30.85 (0.79-1208.15) | 0.92 (0.36-2.36) |
| Maternal ethnicity | |||
| Not Hispanic or Latino | ref | ref | ref |
| Hispanic or Latino | 0.42 (0.13-1.36) | 14.09 (0.14-1427.37) | 0.46 (0.15-1.37) |
| Paternal ethnicity | |||
| Not Hispanic or Latino | ref | ref | ref |
| Hispanic or Latino | 2.19 (0.76-6.37) | 0.23 (0.01-9.98) | 2.26 (0.82-6.25) |
| Education level | |||
| Less than high school | ref | ref | ref |
| High school or GED | 1.32 (0.67-2.61) | 2.30 (0.32-16.64) | 1.36 (0.77-2.39) |
| College | 0.92 (0.38-2.23) | 2.21 (0.20-24.79) | 0.80 (0.39-1.63) |
| Employment | |||
| Employed | ref | ref | ref |
| Unemployed | 0.90 (0.57-1.44) | 1.17 (0.32-4.20) | 0.85 (0.57-1.28) |
| Insurance status | |||
| Public | ref | ref | ref |
| Private | 0.63 (0.28-1.45) | 0.07 (0.00-1.26) | 0.62 (0.29-1.30) |
| None/self-pay | 0.32 (0.08-1.26) | 0.64 (0.02-26.79) | 0.57 (0.19-1.73) |
| Marital status | |||
| Living with partner | ref | ref | ref |
| Not living with partner | 0.84 (0.47-1.50) | 0.12 (0.02-0.80)a | 0.75 (0.45-1.24) |
| Household income | |||
| $0-$12 000 | ref | ref | ref |
| $12 001-$24 000 | 0.69 (0.34-1.40) | 0.04 (0.01-0.31)a | 0.64 (0.35-1.17) |
| $24 001-$50 000 | 1.31 (0.54-3.18) | 0.02 (0.00-0.43)a | 0.86 (0.41-1.82) |
| >$50 000 | 1.26 (0.48-3.32) | 0.01 (0.00-0.29)a | 1.13 (0.48-2.62) |
| Unknown | 2.40 (1.24-4.66)a | 3.77 × 10-11 (0.00-inf) | 1.85 (1.04-3.30)a |
| Age | 0.98 (0.92-1.04) | 0.94 (0.77-1.15) | 0.94 (0.89-0.99)a |
| BMI | 0.98 (0.95-1.01) | 0.91 (0.78-1.05) | 0.98 (0.95-1.01) |
| Clinical phenotypes | |||
| Infection/inflammation | |||
| No evidence | ref | ref | ref |
| Some evidence | 1.18 (0.78-1.79) | 1.83 (0.45-7.40) | 1.12 (0.78-1.62) |
| Decidual hemorrhage | |||
| No evidence | ref | ref | ref |
| Some evidence | 3.64 (2.17-6.12)a | 1.17 (0.19-7.08) | 2.97 (1.88-4.69)a |
| Cervical insufficiency | |||
| No evidence | ref | ref | ref |
| Some evidence | 3.63 (2.00-6.58)a | 2.20 (0.27-17.78) | 3.00 (1.74-5.16)a |
| Placental dysfunction | |||
| No evidence | ref | ref | ref |
| Possible evidence | 1.67 (0.98-2.85) | 0.36 (0.04-3.30) | 1.16 (0.70-1.91) |
| Moderate evidence | 1.07 (0.44-2.61) | 10.70 (2.71-42.22)a | 1.22 (0.71-2.11) |
| Family history | |||
| No evidence | ref | ref | ref |
| Moderate evidence | 0.54 (0.35-1.17) | 0.23 (0.03-2.01) | 0.55 (0.28-1.06) |
| Strong evidence | 1.13 (0.74-1.72) | 2.65 (0.72-9.73) | 1.22 (0.84-1.78) |
| Maternal comorbidities | |||
| No evidence | ref | ref | ref |
| Some evidence | 1.04 (0.64-1.70) | 1.96 (0.54-7.16) | 1.05 (0.69-1.59) |
| Maternal stress | |||
| No evidence | ref | ref | ref |
| Possible evidence | 1.33 (0.74-2.39) | 4.36 (0.93-20.47) | 1.16 (0.70-1.90) |
| Strong evidence | 1.29 (0.62-2.66) | 1.23 (0.16-9.60) | 1.27 (0.71-2.29) |
| Previous obstetric history | |||
| Previous caesarean section | |||
| No | ref | ref | ref |
| Yes | 1.65 (0.97-2.82) | 4.83 (1.01-23.05)a | 1.28 (0.82-2.00) |
| Number of previous deliveries | 1.08 (0.90-1.30) | 2.21 (1.17-4.20)a | 1.25 (1.08-1.45)a |
| Number of abortions | 1.01 (0.79-1.29) | 3.33 (1.54-7.24)a | 1.15 (0.93-1.42) |
| Time since last pregnancy | 1.04 (0.94-1.14) | 1.24 (0.95-1.62) | 1.05 (0.96-1.14) |
| Previous birth weightb | 1.00 (0.99-1.00)a | 1.00 (0.99-1.00)a | 1.00 (0.99-1.00)a |
| Model performance | |||
| Concordance | 0.76 (SE = 0.02) | 0.87 (SE = 0.04) | 0.72 (SE = 0.02) |
| AIC | 1212.706 | 239.2805 | 1593.33 |
| Log-rank test | 115.8 (P < .001)a | 47.22 (P = .05)a | 106.8 (P < .001)a |
Hazard ratios and 95% CIs are presented for each category in regard to the reference group. Owing to a small number of participants in the dataset who had preterm indicated delivery (n = 25), maternal race and paternal race were perfectly collinear for the Black or African American category, which resulted in abnormal hazard ratios and infinite CIs. None of the participants who had an indicated preterm delivery had an unknown income, which led to a hazard ratio approaching zero and an infinite CI. Concordance, AIC values, and log-rank test statistics are presented as model performance measures. Higher concordance, lower AIC, and lower P values for log-rank test are characteristics of a better fit.
AIC: Akaike information criterion; BMI: body mass index; CI: confidence interval; inf: infinity; ref: reference.
Statistically significant value.
The 95% CI for previous birth weight does not cross the line of no effect. The upper CI value is reported as 1.00 due to rounding.