Table 2.
Adverse impact of maternal influenza infection on the birth outcomes, stratified by pregnancy periods
| Control (n = 1,525,972) | Cases with influenza – stratified by pregnancy periods at influenza infection | ||||
|---|---|---|---|---|---|
| Trimester 1† (n = 21,396) | Trimester 2† (n = 9615) | Trimester 3† (n = 6643) | p-value | ||
| Birth weight, mean ± SD (kg)a | 3.216 ± 0.485a | 3.203 ± 0.500a | 3.209 ± 0.496a | 3.194 ± 0.500b | < 0.0001 |
| Preterm birth (n, %)a | 40,798 (2.67) a | 847 (3.96) b | 305 (3.17) c | 253 (3.81) d | < 0.0001 |
| LBW (n, %)a | 56,773 (3.72) a | 974 (4.55) b | 375 (3.90) c | 319 (4.80) d | < 0.0001 |
| Macrosomia (n, %) | 62,135 (4.07) | 848 (3.96) | 404 (4.20) | 280 (4.21) | 0.7035 |
| Logistic regression analysis | |||||
| Preterm birth (n, %) | unadjusted OR (95% CI) | age-adjusted OR (95% CI) | OR adjusted for age, diabetes, hypertension and primiparity (95% CI) | ||
| Influenza (−) | ref | ref | ref | ||
| Influenza at trimester 1 | 1.500 (1.400–1.608) | 1.506 (1.405–1.614) | 1.500 (1.399–1.607) | ||
| Influenza at trimester 2 | 1.193 (1.064–1.337) | 1.200 (1.070–1.346) | 1.190 (1.062–1.335) | ||
| Influenza at trimester 3 | 1.441 (1.271–1.635) | 1.442 (1.271–1.636) | 1.432 (1.262–1.625) | ||
| Influenza at any trimester | 1.411 (1.337–1.490) | 1.416 (1.341–1.495) | 1.408 (1.334–1.487) | ||
| LBW (n, %) | |||||
| Influenza (−) | ref | ref | ref | ||
| Influenza at trimester 1 | 1.234 (1.157–1.317) | 1.238 (1.160–1.321) | 1.234 (1.156–1.316) | ||
| Influenza at trimester 2 | 1.050 (0.947–1.165) | 1.055 (0.951–1.170) | 1.049 (0.945–1.163) | ||
| Influenza at trimester 3 | 1.306 (1.167–1.462) | 1.307 (1.167–1.463) | 1.300 (1.161–1.455) | ||
| Influenza at any trimester | 1.200 (1.141–1.261) | 1.203 (1.145–1.264) | 1.198 (1.140–1.259) | ||
| Macrosomia (n, %) | |||||
| Influenza (−) | ref | ref | ref | ||
| Influenza at trimester 1 | 0.973 (0.908–1.042) | 0.975 (0.910–1.044) | 0.973 (0.908–1.043) | ||
| Influenza at trimester 2 | 1.034 (0.936–1.142) | 1.038 (0.939–1.147) | 1.036 (0.937–1.144) | ||
| Influenza at trimester 3 | 1.037 (0.920–1.169) | 1.037 (0.920–1.169) | 1.035 (0.918–1.166) | ||
| Influenza at any trimester | 1.000 (0.950–1.053) | 1.002 (0.951–1.055) | 1.000 (0.949–1.053) | ||
a The same letters indicate non-significant differences between groups based on Tukey’s multiple comparison test and Chi-square test
† Trimester 1 = 181–280 days from delivery; trimester 2 = 91–180 days from delivery; trimester 3 = 0–90 days from delivery
SD standard deviation; LBW low birth weight