Risk of pregnancy complications and adverse birth outcomes after maternal A(H1N1)pdm09 influenza: a Norwegian population-based cohort study

Table 6 HI-titer in relation to risk of pre-eclampsia, preterm birth, small for gestational age birth, and birth weight in women with pregnancy start before October 1, 2009

	HI-titer = 5, n = 293	HI-titer ≥10, n = 118
Pre-eclampsia
Cases, n (%)	6 (2.0)	2 (1.7)
Crude OR (95% CI)	1 (Ref)	0.82 (0.16, 4.15)
Adjusted OR (95% CI)^a	1 (Ref)	0.98 (0.18, 5.46)
Preterm birth
Cases, n (%)	7 (2.4)	3 (2.5)
Crude OR (95% CI)	1 (Ref)	1.07 (0.27, 4.19)
Adjusted OR (95% CI)^a	1 (Ref)	1.30 (0.31, 5.41)
Small for gestational age birth
Cases, n (%)	17 (5.8)	11 (9.3)
Crude OR (95% CI)	1 (Ref)	1.67 (0.76, 3.68)
Adjusted OR (95% CI)^a	1 (Ref)	1.75 (0.77, 3.97)
Birth weight
10th percentile (g)	3070	2940
Difference in 10th percentile, β (95% CI)^b	1 (Ref)	−159 (−309, −9)
50th percentile (g)	3560	3610
Difference in 50th percentile, β (95% CI)^b	1 (Ref)	−31 (− 158, 96)

^aLogistic regression adjusted for maternal age at birth (continuous), parity (primiparous/multiparous), smoking during pregnancy (yes/no), and time in days between birth and pandemic (continuous). The model was also adjusted for influenza risk group (yes/no) except with pre-eclampsia as the outcome
^bQuantile regression adjusted for maternal age at birth (continuous), parity (primiparous/multiparous), smoking during pregnancy (yes/no), influenza risk group (yes/no), and time in days between birth and pandemic (continuous)

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