Table 1.
Parental and infant characteristics by maternal smoking during pregnancy. Values are numbers (percentages) unless stated otherwise
| Characteristics | All (n=16 80 307) | Maternal smoking during pregnancy | |
|---|---|---|---|
| No (n=13 02 940) | Yes (n=377 367) | ||
| Parental characteristics | |||
| Maternal age (years): | |||
| <20 | 43 776 (2.6) | 25 552 (2.0) | 18 224 (4.8) |
| 20-24 | 350 843 (20.9) | 250 254 (19.2) | 100 589 (26.7) |
| 25-29 | 621 975 (37.0) | 493 401 (37.9) | 128 574 (34.1) |
| 30-34 | 454 709 (27.1) | 367 170 (28.2) | 87 539 (23.2) |
| ≥35 | 209 004 (12.4) | 166 563 (12.8) | 42 441 (11.2) |
| Maternal height (cm): | |||
| <155 | 42 022 (3.0) | 33 239 (3.0) | 8783 (2.9) |
| 155-164 | 511 968 (36.5) | 393 284 (35.9) | 118 684 (38.6) |
| 165-174 | 733 366 (52.2) | 575 759 (52.5) | 157 607 (51.2) |
| ≥175 | 116 506 (8.3) | 93 829 (8.6) | 22 677 (7.4) |
| Missing | 276 445 (16.5) | 206 829 (15.9) | 69 616 (18.4) |
| Maternal body mass index (kg/m2): | |||
| <18.5 | 58 540 (5.0) | 40 146 (4.4) | 18 394 (7.3) |
| 18.5–24.9 | 827 417 (71.0) | 654 244 (71.6) | 173 173 (68.9) |
| 25.0–29.9 | 213 032 (18.3) | 168 417 (18.4) | 44 615 (17.8) |
| ≥30.0 | 65 680 (5.6) | 50 616 (5.5) | 15 064 (6.0) |
| Missing | 515 638 (30.7) | 389 517 (29.9) | 126 121 (33.4) |
| Maternal parity: | |||
| Nulliparous | 693 810 (41.3) | 539 717 (41.4) | 154 093 (40.8) |
| 1 | 607 805 (36.2) | 480 972 (36.9) | 126 833 (33.6) |
| 2 | 265 846 (15.8) | 201 569 (15.5) | 64 277 (17.0) |
| ≥3 | 112 846 (6.7) | 80 682 (6.2) | 32 164 (8.5) |
| Maternal education: | |||
| Compulsory up to 9 years | 287 413 (17.6) | 166 743 (13.1) | 120 670 (32.7) |
| Secondary | 893 398 (54.6) | 684 956 (54.0) | 208 442 (56.5) |
| Post-secondary | 456 668 (27.9) | 417 115 (32.9) | 39 553 (10.7) |
| Missing | 42 828 (2.5) | 34 126 (2.6) | 8702 (2.3) |
| Maternal occupational classification*: | |||
| Low | 971 155 (57.8) | 704 671 (54.1) | 266 484 (70.6) |
| Intermediate | 328 018 (19.5) | 270 283 (20.7) | 57 735 (15.3) |
| High | 311 077 (18.5) | 278 412 (21.4) | 32 665 (8.7) |
| Missing | 70 057 (4.2) | 49 574 (3.8) | 20 483 (5.4) |
| Maternal marital status: | |||
| Not married | 651 902 (39.3) | 474 840 (36.9) | 177 062 (47.5) |
| Married | 937 856 (56.6) | 768 373 (59.8) | 169 483 (45.4) |
| Divorced or widowed | 68 618 (4.1) | 42 081 (3.3) | 26 537 (7.1) |
| Missing | 21 931 (1.3) | 17 646 (1.4) | 4285 (1.1) |
| Paternal education: | |||
| Compulsory up to 9 years | 336 267 (20.7) | 223 267 (17.7) | 113 000 (31.4) |
| Secondary | 852 749 (52.5) | 649 389 (51.4) | 203 360 (56.4) |
| Post-secondary | 434 226 (26.8) | 390 306 (30.9) | 43 920 (12.2) |
| Missing | 57 065 (3.4) | 39 978 (3.1) | 17 087 (4.5) |
| Paternal occupational classification: | |||
| Low | 1 037 614 (61.8) | 762 732 (58.5) | 274 882 (72.8) |
| Intermediate | 203 004 (12.1) | 168 129 (12.9) | 34 875 (9.2) |
| High | 373 226 (22.2) | 328 080 (25.2) | 45 146 (12.0) |
| Missing | 66 463 (4.0) | 43 999 (3.4) | 22 464 (6.0) |
| Infant characteristics | |||
| Sex: | |||
| Boy | 863 638 (51.4) | 669 223 (51.4) | 194 415 (51.5) |
| Girl | 816 669 (48.6) | 633 717 (48.6) | 182 952 (48.5) |
| Mean (SD) birth weight (g) | 3538 (556) | 3582 (548) | 3384 (559) |
| Missing | 0.4 (5899) | 0.3 (4490) | 0.4 (1409) |
| Mean (SD) gestational age at birth (weeks) | 39.4 (1.8) | 39.4 (1.8) | 39.2 (1.9) |
| Missing | 0.1 (996) | 0.0 (633) | 0.1 (363) |
| Small for gestational age (<10th centile): | |||
| No | 1 509 017 (90.2) | 1 192 715 (91.9) | 316 302 (84.2) |
| Yes | 164 425 (9.8) | 105 124 (8.1) | 59 301 (15.8) |
| Missing | 6865 (0.4) | 5101 (0.4) | 1764 (0.5) |
For all variables, numbers (percentages) are only given for singleton births with no missing values to facilitate comparison by maternal smoking during pregnancy.
*
Defined using an approximation of the European Socio-economic Classification (ESeC) and grouped into lower, intermediate, and higher level professions.36