Table 1.
Characteristics of the study populations, the NT (1986–2017), Qld (2007–2018), NSW (1994–2018).
| NT | Qld | NSW | ||||
|---|---|---|---|---|---|---|
| Characteristic | Exposed n (%) |
Comparison n (%) |
Exposed n (%) |
Comparison n (%) |
Exposed n (%) |
Comparison n (%) |
| Mothers | ||||||
| Total sample 1 | 4083 | 8607 | 513 | 2876 | 144 | 4871 |
| Country of birth | ||||||
| Australia | 3529 (87) | 6825 (80) | 463 (90) | 2642 (92) | 130 (90) | 4522 (93) |
| Overseas | 543 (13) | 1720 (20) | 50 (10) | 234 (8) | 14 (10) | 349 (7) |
| Missing/unknown | 11 | 62 | 0 | 0 | 0 | 0 |
| Indigenous status 2 | ||||||
| No | 2916 (71) | 6795 (79) | 424 (83) | 2707 (94) | 139 (97) | 4651 (96) |
| Yes | 1167 (29) | 1806 (21) | 89 (17) | 169 (6) | ≤5 | 206 (4) |
| Missing/unknown | 0 | 6 | 0 | 0 | ≤5 | 14 |
| Pregnancies | ||||||
| Total sample 1 | 5606 | 11,364 | 665 | 3989 | 188 | 7287 |
| Year of birth | ||||||
| 1985–1994 | 1731 (31) | 2457 (22) | N/A | N/A | ≤5 | 264 (4) |
| 1995–2004 | 1724 (31) | 3466 (31) | N/A | N/A | 41 (22) | 2754 (38) |
| 2005–2014 | 1736 (31) | 4157 (37) | 445 (67) | 2681 (67) | 100 (53) | 3033 (42) |
| 2015–latest | 415 (7) | 1283 (11) | 220 (33) | 1308 (33) | 44 (23) | 1236 (17) |
| Missing/unknown | 0 | 1 | 0 | 0 | ≤5 | 0 |
| Mother’s age (at baby’s birth) 3 | ||||||
| <20 | 606 (11) | 925 (8) | 87 (13) | 360 (9) | ≤5 | 332 (5) |
| 20–24 | 1344 (24) | 2182 (19) | 201 (30) | 961 (24) | 22 (12) | 1284 (18) |
| 25–29 | 1714 (31) | 3187 (28) | 194 (29) | 1281 (32) | 60 (32) | 2374 (33) |
| 30–34 | 1315 (23) | 3252 (29) | 121 (18) | 865 (22) | 60 (32) | 2105 (29) |
| 35–39 | 526 (9) | 1534 (14) | 62 (9) | 522 (13) | 39 (21) | 976 (13) |
| 40+ | 101 (2) | 281 (2) | N/A | N/A | 6 (3) | 215 (3) |
| Missing/unknown | 0 | 3 | 0 | 0 | ≤5 | 1 |
| Gestational age (weeks) | ||||||
| ≤36 | 455 (8) | 929 (8) | 61 (9) | 314 (8) | 15 (8) | 399 (5) |
| 37–40 | 4298 (77) | 9046 (80) | 524 (79) | 3209 (80) | 139 (74) | 5553 (76) |
| 41+ | 827 (15) | 1300 (12) | 80 (12) | 465 (12) | 34 (18) | 1332 (18) |
| Missing | 26 | 89 | 0 | 1 | 0 | 3 |
| Baby sex | ||||||
| Female | 2688 (48) | 5607 (49) | 329 (49) | 1899 (48) | 94 (50) | 3552 (49) |
| Male | 2916 (52) | 5753 (51) | 336 (51) | 2090 (52) | 94 (50) | 3735 (51) |
| Missing/unknown | 2 | 4 | 0 | 0 | 0 | 0 |
| Maternal parity | ||||||
| No prior birth | 2087 (37) | 4683 (41) | 190 (29) | 1033 (26) | 72 (38) | 2595 (36) |
| One prior birth | 1676 (30) | 3501 (31) | 160 (24) | 1054 (26) | 69 (37) | 2436 (33) |
| ≥Two prior births | 1840 (33) | 3156 (28) | 315 (47) | 1902 (48) | 47 (25) | 2246 (31) |
| Missing/unknown | 3 | 24 | 0 | 0 | 0 | 10 |
| Marital status (at birth) ⁴ | ||||||
| Married/de facto | 3610 (65) | 7156 (64) | 484 (73) | 3133 (79) | 12 (80) | 960 (87) |
| Other | 1959 (35) | 4014 (36) | 181 (27) | 856 (21) | 3 (20) | 148 (13) |
| Missing/unknown | 37 | 194 | 0 | 0 | 173 | 6179 |
| Pre-pregnancy BMI ⁵ | ||||||
| 0 -< 18.5 | 13 (3) | 48 (3) | 25 (4) | 210 (5) | ≤5 | 34 (4) |
| 18.5 -< 25 | 209 (43) | 772 (53) | 253 (41) | 1596 (41) | 12 (41) | 404 (44) |
| 25 -< 30 | 139 (29) | 393 (27) | 160 (26) | 1012 (26) | ≤10 | 244 (27) |
| ≥30 | 125 (26) | 257 (17) | 180 (29) | 1072 (28) | 9 (31) | 228 (25) |
| Missing/unknown | 5120 | 9894 | 47 | 99 | 159 | 6377 |
| Smoking during pregnancy ⁶ | ||||||
| No | 2293 (68) | 5623 (75) | 471 (71) | 2959 (74) | 158 (84) | 5909 (81) |
| Yes | 1092 (32) | 1912 (25) | 191 (29) | 1025 (26) | 30 (16) | 1355 (19) |
| Missing/unknown | 2221 | 3829 | 3 | 5 | 0 | 23 |
| IRSD ⁷ decile | ||||||
| 1 | N/A | N/A | 0 | 97 (2) | N/A | N/A |
| 2 | N/A | N/A | 0 | 599 (15) | N/A | N/A |
| 3 | N/A | N/A | 665 (100) | 2966 (74) | N/A | N/A |
| 4 | N/A | N/A | 0 | 327 (8) | N/A | N/A |
| 5–10 | N/A | N/A | 0 | 0 | N/A | N/A |
| Remoteness | ||||||
| Major city | N/A | N/A | 0 | 0 | N/A | N/A |
| Inner regional | N/A | N/A | 0 | 0 | N/A | N/A |
| Outer regional | N/A | N/A | 0 | 43 (1) | N/A | N/A |
| Remote | N/A | N/A | 665 (100) | 3946 (99) | N/A | N/A |
| Very remote | N/A | N/A | 0 | 0 | N/A | N/A |
N/A: not applicable; Denominators for proportions exclude missing values; Cells have been suppressed to avoid reporting cell numbers with size ≤ 5; Percentages were rounded to integer values. Data sources: NSW Perinatal Data Collection (1994–2018), the NT Perinatal Data Collection (1986–2017), Qld Perinatal Data Collection (2007–2018). 1 Mothers can move between exposure and comparison areas; therefore, totals are not for unique mothers. 2 Mothers who had two or fewer perinatal records were coded as Aboriginal and Torres Strait Islander if the mother identified as Aboriginal and/or Torres Strait Islander at least once. Mothers who had more than two pregnancies were coded as Aboriginal and Torres Strait Islander if the mother identified as Aboriginal and/or Torres Strait Islander at least two times. 3 Mother’s age is top-coded at 35 years in the Qld Perinatal Data Collection (PDC). 4 Marital status is only available from 1994–1997 in the NSW PDC and thus was not used as a covariate due to insufficient data. 5 BMI is only available from 2014–2017 in the NT PDC and 2016–2018 in the NSW PDC and thus was not used as a covariate in these states due to insufficient data. 6 Smoking during pregnancy is only available from 1996 in the NT PDC. 7 Index of Relative Socioeconomic Disadvantage (IRSD) based on Statistical Area Level 2 of the mother’s usual address coded according to the Australian Bureau of Statistics Australian Statistical Geography Standard (ASGS) 2011 Version for births up to 2016/2017 and ASGS 2016 Version for births from 2017/2018. IRSD decile and remoteness area were not available in the NT and QLD PDCs.