Table 2.
Basic descriptive characteristics of twins and singletons born in Denmark during 1986-8 stratified for presence of ninth grade test score. Values are numbers (percentages) unless stated otherwise
| Characteristics | Singletons* | Twins | |||
|---|---|---|---|---|---|
| Test score missing | Test score available | Test score missing | Test score available | ||
| No in sample | 1221 | 6575 | 545 | 2866 | |
| Mean (SD) median No of days in hospital (age <1 year) | 9.0 (16.5) 5 | 6.2 (6.9) 5 | 22.8 (26.7) 13 | 14.8 (16.1) 9 | |
| Mean (SD) median No of days in hospital (age 1-14 years) | 6.6 (24.3) 1 | 2.9 (11.2) 0 | 7.4 (23.9) 1 | 2.9 (8.5) 0 | |
| Hospital stay (days) age 1-14 years: | |||||
| None | 524 (42.9) | 3540 (53.8) | 192 (35.2) | 1481 (51.7) | |
| 1-2 | 277 (22.7) | 1453 (22.1) | 153 (28.1) | 674 (23.5) | |
| 3-30 | 369 (30.2) | 1494 (22.7) | 172 (31.6) | 669 (23.3) | |
| 31-60 | 30 (2.5) | 56 (0.9) | 15 (2.8) | 31 (1.1) | |
| ≥61 | 21 (1.7) | 32 (0.5) | 13 (2.4) | 11 (0.4) | |
| Male | 738 (60.4) | 3292 (50.1) | 306 (56.1) | 1409 (49.2) | |
| Birth weight: | |||||
| Mean (SD) birth weight (g) | 3372 (603); n=1214 | 3463 (523); n=6557 | 2369 (600); n=545 | 2574 (530); n=2861 | |
| ≤1500 | 1.4; n=17 | 0.3; n=18 | 9.7; n=53 | 3.5; n=100 | |
| 1500-2000 | 1.3; n=16 | 0.7; n=48 | 16.1; n=88 | 11.1; n=318 | |
| 2000-2500 | 3.9; n=47 | 2.6; n=172 | 31.7; n=173 | 27.9; n=799 | |
| 2500-3000 | 16.8; n=204 | 14.1; n=926 | 29.4; n=160 | 37.8; n=1082 | |
| 3000-3500 | 36.4; n=442 | 36.9; n=2420 | 12.1; n=66 | 16.6; n=474 | |
| 3500-4000 | 27.8; n=337 | 32.0; n=2098 | 0.9; n=5 | 3.0; n=85 | |
| >4000 | 12.4; n=151 | 13.3; n=875 | 0; n=0 | 0.1; n=3 | |
| Mean (SD) gestational age (weeks) | 39.5 (2.0); n=1214 | 39.7 (1.6); n=6545 | 36.2 (3.0); n=545 | 37.1 (2.3); n=2861 | |
| Mean (SD) maternal age | 27.1 (5.0); n=1221 | 28.0 (4.7); n=6574 | 28.5 (4.6); n=545 | 29.1 (4.5); n=2866 | |
| Mean (SD) paternal age | 30.2 (6.2); n=1207 | 30.9 (5.7); n=6531 | 31.6 (5.7); n=544 | 31.9 (5.7); n=2858 | |
| Mean (SD) maternal education† | 1.24 (1.59); n=1183 | 1.74 (1.73); n=6446 | 1.26 (1.69); n=528 | 1.81 (1.78); n=2790 | |
| Mean (SD) paternal education† | 1.14 (1.47); n=1115 | 1.66 (1.79); n=6215 | 1.30 (1.72); n=508 | 1.75 (1.84); n=2725 | |
Denominators vary owing to missing values.
*5% random sample of Danish birth cohorts, 1986-8.
†See footnote to table 1.