Table 1. Maternal characteristics at study entry (first birth in the included cohort)* for the within-women analysis (n = 1,233,688).
| Characteristics | Australia | Finland | Norway | California |
|---|---|---|---|---|
| N (%) | N (%) | N (%) | N (%) | |
| Total women | 306,639 | 202,252 | 219,770 | 505,027 |
| Parity** | ||||
| 1 | 273,911 (89.3) | 186,615 (92.3) | 205,117 (93.4) | 495,390 (98.1) |
| 2 | 20,649 (6.7) | 9,352 (4.6) | 10,851 (4.9) | 9,222 (1.8) |
| ≥3 | 12,079 (3.9) | 6,285 (3.10) | 3,802 (1.7) | 415 (0.1) |
| Maternal age (years) | ||||
| <20 | 12,764 (4.2) | 2,749 (1.4) | 2,194 (1.0) | 50,034(9.9) |
| 20–24 | 75,219 (24.5) | 48,734 (24.1) | 51,674 (23.5) | 177,492 (35.2) |
| 25–29 | 109,210 (35.6) | 86,098 (42.6) | 98,248 (44.7) | 148,185 (20.3) |
| 30–34 | 85,271 (27.8) | 52,316 (25.9) | 56,904 (25.9) | 100,803 (20.0) |
| 35–39 | 22,686 (7.4) | 11,442 (5.7) | 10,215 (4.65) | 26,882 (5.3) |
| ≥40 | 1,489 (0.5) | 913 (0.5) | 535 (0.2) | 1,631 (0.3) |
| Birth year | ||||
| 1980–1989 | 29,804 (9.7) | 9,400 (4.6) | 54,220 (24.7) | - |
| 1990–1999 | 79,256 (25.8) | 87,752 (43.4) | 79,037 (36.0) | 199,063 (30.5) |
| 2000–2009 | 147,576 (48.1) | 75,221 (37.2) | 70,198 (31.9) | 290,640 (57.5) |
| 2010 or later | 50,003 (16.3) | 29,879 (14.8) | 16,315 (7.4) | 15,324 (3.0) |
| Socioeconomic index for areas (quintile) | ||||
| Lowest | 71,259 (23.2) | N/A | N/A | N/A |
| Low | 62,479 (20.4) | N/A | N/A | N/A |
| Middle | 56,814 (18.5) | N/A | N/A | N/A |
| High | 52,143 (17.0) | N/A | N/A | N/A |
| Highest | 47,595 (15.5) | N/A | N/A | N/A |
| Other/unknown/missing | 16,349 (5.3) | N/A | N/A | N/A |
| Maternal occupation | ||||
| Upper-white collar worker | N/A | 22,675 (13.9) | N/A | N/A |
| Lower-white collar worker | N/A | 74,147 (36.7) | N/A | N/A |
| Blue collar worker | N/A | 32,702 (16.2) | N/A | N/A |
| Other or unknown | N/A | 67,728 (33.5) | N/A | N/A |
| Highest level of maternal education | ||||
| Some high school or less | N/A | N/A | N/A | 99,696 (19.7) |
| High school diploma/Equivalent | N/A | N/A | N/A | 169,195 (33.5) |
| Some college | N/A | N/A | N/A | 120,038 (23.8) |
| College graduate or more | N/A | N/A | N/A | 109,357 (21.7) |
| Missing | N/A | N/A | N/A | 6,741 (1.3) |
| Number of IPIs | ||||
| 2 | 220,175 (71.8) | 144,141(71.3) | 177,636 (80.8) | 385,449 (76.3) |
| ≥3 | 86,464 (28.2) | 58,111 (28.7) | 42,134 (19.2) | 119,578 (23.7) |
*Study entry is defined as the earliest birth (second birth) at which the women appeared within the included cohort during the study period.
**Women parity at the birth when entering to the cohort (i.e the second birth after interval). IPI -_Interpregnancy intervals. N/A—data were not available in these countries.