Table 1.
Subgroup analysis for maternal extreme low ambient temperature exposure and preterm birth.
| Subgroup | N of study | Pooled RR (95% CI) | I2 (%) |
| Exposure period | |||
| Preconception | 4 | 1.03 (0.89, 1.20) | 83 |
| First trimester | 6 | 0.99 (0.95, 1.04) | 89 |
| Second trimester | 6 | 0.98 (0.93, 1.03) | 92 |
| Third trimester | 5 | 1.16 (1.12, 1.20) | 39 |
| Entire pregnancy | 3 | 1.09 (0.99, 1.20) | 77 |
| Birth month | 7 | 1.15 (1.00, 1.33) | 89 |
| Birth week | 9 | 1.09 (1.03, 1.16) | 83 |
| At birth | 5 | 1.34 (1.15, 1.57) | 95 |
| Temperature grade of exposure (%) | |||
| <1st | 9 | 1.14 (1.06, 1.23) | 77 |
| <5th | 8 | 1.05 (0.99, 1.11) | 95 |
| <10th | 3 | 1.13 (0.94, 1.36) | 92 |
| <25th | 3 | 1.07 (0.77, 1.48) | 86 |
| Climate | |||
| Cold area | 4 | 0.96 (0.92, 1.01) | 77 |
| Medium area | 7 | 1.15 (1.02, 1.30) | 80 |
| Hot area | 8 | 1.09 (1.00, 1.18) | 97 |
| Race | |||
| Asian | 10 | 1.13 (1.06, 1.20) | 90 |
| Black | 1 | 1.37 (1.02, 1.83) | – |
| Caucasian | 6 | 1.02 (0.96, 1.08) | 60 |
| Type of exposure | |||
| Exposure levels of Tmean | 14 | 1.07 (1.02, 1.12) | 94 |
| Exposure levels of Tmin | 2 | 1.13 (0.84, 1.53) | 77 |
| Exposure levels of Tmax | 1 | 0.97 (0.86, 1.09) | – |
| Cold spell | 1 | 1.40 (1.17, 1.67) | – |
| 2 days/mouth increase of extreme cold | 1 | 1.15 (1.07, 1.25) | – |
| Methodological model | |||
| Distributed lag non-linear model | 9 | 1.15 (1.03, 1.30) | 91 |
| Cox proportional hazard regression | 4 | 1.07 (1.03, 1.11) | 62 |
| Logistic regression model | 2 | 1.06 (0.91, 1.24) | 91 |
| Generalized additive model | 1 | 1.57 (1.14, 2.17) | – |
| Chi-squared test | 1 | 1.14 (1.10, 1.19) | – |
| Poisson regression | 1 | 0.96 (0.94, 0.99) | – |
CI: Confidence interval; RR: Risk ratio; Tmean: Daily mean temperature; Tmin: Daily minimum temperature; Tmax: Daily maximum temperature; −: Not available.