Distribution of background characteristics
| Background characteristics | Registry cohort n=474 402 N (%) | MoBa n=75 797 N (%) |
| Maternal age at delivery, years | ||
| <25 | 79 803 (16.8) | 8791 (11.6) |
| 25–29 | 155 866 (32.9) | 25 367 (33.5) |
| 30–34 | 159 001 (33.5) | 29 024 (38.3) |
| 35 and older | 79 732 (16.8) | 12 615 (16.6) |
| Maternal parity | ||
| 0 | 193 503 (40.8) | 33 059 (43.6) |
| 1 | 170 737 (36.0) | 27 291 (36.0) |
| 2 | 78 163 (16.5) | 12 009 (15.8) |
| 3+ | 31 999 (6.8) | 3438 (4.5) |
| Maternal education | ||
| Less than high school | NA | 6698 (8.8) |
| High school | NA | 23 485 (31.0) |
| Up to 4 years of college | NA | 30 223 (39.9) |
| >4 years of college | NA | 15 018 (19.8) |
| Missing | NA | 373 (0.5) |
| Maternal prepregnancy body mass index | ||
| <18.5 | NA | 2313 (3.1) |
| 18.5–24.9 | NA | 47 768 (63.0) |
| 25–29.9 | NA | 16 391 (21.6) |
| 30 or higher | NA | 7122 (9.5) |
| Missing | NA | 2203 (2.9) |
| Maternal folic acid supplement intake | ||
| No | 271 981 (57.3) | 16 020 (21.1) |
| Started before pregnancy | 76 333 (16.1) | 30 835 (40.7) |
| Started during pregnancy | 124 732 (26.3) | 28 942 (38.2) |
| Missing | 1356 (0.3) | 0 (0) |
| Maternal smoking during pregnancy | ||
| No | 317 226 (66.9) | 56 564 (74.6) |
| Only at the beginning of pregnancy (register)/quit by 18 weeks (MoBa) | 27 929 (5.9) | 10 811 (14.3) |
| Still smoking end of pregnancy (register)/smoked after 18 weeks (MoBa) | 48 855 (10.3) | 8057 (10.6) |
| Missing | 80 392 (17.0) | 365 (0.5) |
| Maternal current asthma | ||
| No | 454 114 (95.7) | 73 220 (96.6) |
| Yes | 20 288 (4.3) | 2577 (3.4) |
| Maternal pre-eclampsia | ||
| No | 456 100 (96.1) | 72 914 (96.2) |
| Yes | 18 302 (3.9) | 2883 (3.8) |
| Child gender | ||
| Male | 243 413 (51.3) | 38 811 (51.2) |
| Female | 230 989 (48.7) | 36 986 (48.8) |
| Child preterm birth (<37 gestational weeks) | ||
| No | 449 218 (94.7) | 71 849 (94.8) |
| Yes | 25 184 (5.3) | 3646 (4.8) |
| Missing | 0 (0) | 302 (0.4) |
| Child low birth weight (<2500 g) | ||
| No | 458 572 (96.7) | 73 611 (97.1) |
| Yes | 15 830 (3.3) | 2146 (2.8) |
| Missing | 0 (0) | 40 (0.1) |
| Child delivered by caesarean section | ||
| No | 404 328 (85.2) | 65 288 (86.1) |
| Yes | 70 074 (14.8) | 10 509 (13.9) |
MoBa, Norwegian Mother and Child Cohort Study.