Table 2 Adverse impact of maternal influenza infection on the birth outcomes, stratified by pregnancy periods

Birth weight, mean ± SD (kg)^a

3.216 ± 0.485^a

3.203 ± 0.500^a

3.209 ± 0.496^a

3.194 ± 0.500^b

Preterm birth (n, %)^a

40,798 (2.67) ^a

847 (3.96) ^b

305 (3.17) ^c

253 (3.81) ^d

LBW (n, %)^a

56,773 (3.72) ^a

974 (4.55) ^b

375 (3.90) ^c

319 (4.80) ^d

Macrosomia (n, %)

62,135 (4.07)

848 (3.96)

404 (4.20)

280 (4.21)

Preterm birth (n, %)

unadjusted OR (95% CI)

age-adjusted OR (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)

  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)

  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)