Examination of familial confounding in population of full siblings discordant for gestational age born 1974–2017 in Sweden
| Gestational age | Total population (n=3 572 845) | Sibling population (n=2 853 886) | Exposure-discordant siblings (n=677 647) | |||
| Cases | HR (95% CI) | Cases | HR (95% CI) | Cases | HR (95% CI) | |
| Extremely preterm (23–27 weeks) | 364 | 11.21 (10.18 to 12.44) | 268 | 11.96 (10.60 to 13.49) | 215 | 13.91 (9.04 to 21.40) |
| Very preterm (28–31 weeks) | 600 | 4.79 (4.42 to 5.20) | 430 | 4.87 (4.42 to 5.36) | 349 | 4.89 (3.90 to 6.13) |
| Moderate to late preterm (32–36 weeks) | 2681 | 2.18 (2.09 to 2.27) | 1961 | 2.15 (2.05 to 2.25) | 1628 | 1.84 (1.69 to 2.00) |
| Term (37–41 weeks) | 20 860 | Ref | 15 891 | Ref | 2793 | Ref |
| Post-term (42–45 weeks) | 2091 | 1.09 (1.05 to 1.14) | 1498 | 1.08 (1.03 to 1.14) | 1325 | 1.21 (1.11 to 1.31) |
The Cox regression model with child’s age at follow-up as underlying time-scale, adjusted for birth year (5-year intervals from 1974 to 2015 and from 2015 to 2017), maternal age and paternal age (<20, 5-year intervals from 20 to 39 and ≥40).
ref, reference group for all HRs.