Table 2. Association of Season During Gestation and Season of Pregnancy Onset With Risk of Preterm Birtha.
| Season | Births, No. (fetal weeks of follow-up, millions) | Hazard ratio (95% CI) | |
|---|---|---|---|
| Unadjusted | Adjustedb | ||
| Gestation | |||
| Extremely preterm birth | |||
| Winter | 441 (1.7) | 1 [Reference] | 1 [Reference] |
| Spring | 531 (1.9) | 1.10 (0.98-1.25) | 1.11 (0.97-1.26) |
| Summer | 507 (1.7) | 1.12 (0.99-1.27) | 1.15 (1.02-1.31) |
| Autumn | 530 (1.7) | 1.22 (1.08-1.38) | 1.25 (1.10-1.42) |
| Very preterm birth | |||
| Winter | 1093 (1.1) | 1 [Reference] | 1 [Reference] |
| Spring | 1265 (1.2) | 1.08 (1.00-1.17) | 1.08 (0.99-1.17) |
| Summer | 1220 (1.2) | 1.07 (0.99-1.16) | 1.06 (0.98-1.15) |
| Autumn | 1168 (1.1) | 1.13 (1.04-1.22) | 1.13 (1.04-1.22) |
| Moderately preterm birth | |||
| Winter | 7537 (1.4) | 1 [Reference] | 1 [Reference] |
| Spring | 7776 (1.5) | 0.98 (0.95-1.01) | 0.98 (0.95-1.01) |
| Summer | 8354 (1.5) | 1.01 (0.98-1.04) | 1.00 (0.96-1.03) |
| Autumn | 7717 (1.3) | 1.05 (1.02-1.09) | 1.05 (1.02-1.09) |
| Pregnancy onset | |||
| Extremely preterm birth | |||
| Winter | 503 (1.7) | 1 [Reference] | 1 [Reference] |
| Spring | 545 (1.6) | 1.11 (0.99-1.26) | 1.12 (0.99-1.27) |
| Summer | 443 (1.7) | 0.87 (0.76-0.99) | 0.87 (0.77-0.99) |
| Autumn | 518 (1.8) | 0.96 (0.85-1.09) | 0.96 (0.85-1.08) |
| Very preterm birth | |||
| Winter | 1192 (1.1) | 1 [Reference] | 1 [Reference] |
| Spring | 1178 (1.1) | 1.01 (0.93-1.10) | 1.02 (0.94-1.10) |
| Summer | 1141 (1.1) | 0.94 (0.87-1.02) | 0.94 (0.87-1.02) |
| Autumn | 1235 (1.1) | 0.96 (0.89-1.04) | 0.96 (0.88-1.04) |
| Moderately preterm birth | |||
| Winter | 7803 (1.4) | 1 [Reference] | 1 [Reference] |
| Spring | 7770 (1.3) | 1.02 (0.99-1.05) | 1.02 (0.99-1.05) |
| Summer | 7622 (1.4) | 0.95 (0.92-0.99) | 0.96 (0.93-0.99) |
| Autumn | 8180 (1.5) | 0.97 (0.94-1.00) | 0.97 (0.94-1.00) |
a
Among 1 136 143 pregnancies in Denmark with onset between January 1, 1997, and December 31, 2016.
b
Adjusted for maternal employment status (employed, employed in a management position, self-employed, or unemployed and receiving public assistance), family structure (married, single, or living with partner), maternal educational level (primary, secondary, postsecondary, or vocational school), household income (quartile 1, 2, 3, or 4), maternal place of birth (Denmark, Europe, or other), location of maternal residence in Denmark at onset of pregnancy (capital region, middle region, northern region, sea land, or southern region) and the calendar year, and maternal age in years as restricted cubic splines.