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Table 1 Potential confounding factors in relation to maternal smoking during pregnancy in controls

From: Prenatal smoking exposure and neuropsychiatric comorbidity of ADHD: a finnish nationwide population-based cohort study

   No smoking Smoking only in first trimester Smoking after first trimester p χ 2
Maternal age at birth ≤19 years 529 (1.7 %) 48 (7.9 %) 322 (6.4 %) p < 0.001 647.55
20–29 years 15,660 (50.1 %) 365 (59.8 %) 2836 (56.2 %)
30–39 years 14,195 (45.4 %) 186 (30.5 %) 1762 (34.9 %)
≥40 years 886 (2.8 %) 11 (1.8 %) 123 (2.4 %)
Paternal age at birtha ≤19 years 106 (0.3 %) 11 (1.9 %) 95 (1.9 %) p < 0.001 413.72
20–29 years 11,414 (36.8 %) 297 (49.9 %) 2255 (45.9 %)
30–39 years 16,485 (53.1 %) 247 (41.5 %) 2160 (44.0 %)
≥40 years 3052 (9.8 %) 40 (6.7 %) 404 (8.2 %)
History of maternal psychopathology Yes 3321 (10.6 %) 92 (15.1 %) 927 (18.4 %) p < 0.001 258.83
No 27,949 (89.4 %) 518 (84.9 %) 4116 (81.6 %)
History of maternal substance abuse Yes 357 (1.1 %) 22 (3.6 %) 368 (7.3 %) p < 0.001 837.97
No 30,913 (98.9 %) 588 (96.4 %) 4675 (92.7 %)
History of paternal psychopathologya Yes 3387 (10.9 %) 109 (18.3 %) 1101 (22.4 %) p < 0.001 528.64
No 27,670 (89.1 %) 486 (81.7 %) 3813 (77.6 %)
Previous births 0 12,557 (40.2 %) 381 (62.5 %) 2139 (42.4 %) p < 0.001 129.23
≥1 18,713 (59.8 %) 229 (37.5 %) 2904 (57.6 %)
Weight for gestational ageb <-2 SD 716 (2.3 %) 16 (2.6 %) 285 (5.7 %) p < 0.001 219.66
-2 to +2 SD 29,330 (94.1 %) 571 (93.9 %) 4641 (92.4 %)
≥ + 2 SD 1141 (3.7 %) 21 (3.5 %) 95 (1.9 %)
Maternal SES Upper white collar 5325 (17.0 %) 40 (6.6 %) 263 (5.2 %) p < 0.001 1179.59
Lower white collar 14,404 (46.1 %) 234 (38.4 %) 1890 (37.5 %)
Blue collar 4809 (15.4 %) 161 (26.4 %) 1552 (30.8 %)
Others 4663 (14.9 %) 129 (21.2 %) 901 (17.9 %)
Missing 2069 (6.6 %) 46 (7.5 %) 437 (8.7 %)
Maternal immigrant status Not born in Finland 491 (1.6 %) 13 (2.1 %) 38 (0.8 %) p < 0.001 21.91
Born in Finland 30,779 (98.4 %) 597 (97.9 %) 5005 (99.2 %
Paternal immigrant statusa Not born in Finland 557 (1.8 %) 9 (1.5 %) 85 (1.7 %) p < 0.841 0.35
Born in Finland 30,500 (98.2 %) 586 (98.5 %) 4829 (98.3 %)
Apgar score at 1 minc <6 1151 (3.7 %) 30 (4.9 %) 195 (3.9 %) p < 0.229 2.95
7–10 30,086 (96.3 %) 577 (95.1 %) 4841 (96.1 %)
Gestational age d 32 weeks 195 (0.6 %) 3 (0.5 %) 47 (1.0 %) p < 0.029 14.11
3336 weeks 1171 (3.8 %) 19 (3.1 %) 218 (4.3 %)
3741 weeks 28,407 (91.0 %) 553 (90.8 %) 4546 (90.5 %)
42 weeks 1431 (4.6 %) 34 (5.6 %) 212 (4.2 %)
  1. adata missing: 357 subjects
  2. bdata missing: 145 subjects
  3. cdata missing 43 subjects
  4. ddata missing 87 subjects