Table 1.
Frequency distribution and mean (SD) IQ at age 7 and 9, by categories of birth and childhood characteristics, separately for singletons and twins. Values are numbers (percentages) of children unless otherwise indicated
|
Singletons
|
Twins
|
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|---|---|---|---|---|---|---|
|
Mean IQ (SD)
|
Mean IQ (SD)
|
|||||
| Variable | No (%) | Age 7 | Age 9 | No (%) | Age 7 | Age 9 |
| Overall | 9832 (100.0) | 108.2 (15.7) | 112.1 (16.8) | 236 (100.0) | 101.3 (15.8) | 105.1 (17.6) |
| Sex: | ||||||
| Female | 4695 (47.8) | 108.6 (15.8) | 112.8 (16.0) | 106 (44.9) | 99.8 (15.5) | 104.3 (15.5) |
| Male | 5137 (52.2) | 107.9 (15.6) | 111.5 (17.5) | 130 (55.1) | 102.6 (16.0) | 105.8 (19.2) |
| Heterogeneity † | 0.03 | <0.001 | 0.18 | 0.52 | ||
| Birth weight in lb: | ||||||
| <5.5 | 410 (4.2) | 103.3 (15.4) | 107.5 (17.3) | 111 (47.0) | 97.3 (14.5) | 100.9 (15.8) |
| 5.5 to 6.0 | 1620 (16.5) | 106.3 (15.5) | 110.8 (16.2) | 69 (29.2) | 103.5 (15.8) | 107.9 (18.7) |
| 6.5 | 1589 (16.2) | 107.7 (15.8) | 111.3 (16.8) | 24 (10.2) | 107.5 (15.6) | 109.0 (16.6) |
| 7.0 to 7.5 | 3627 (36.9) | 108.6 (15.7) | 112.6 (17.1) | 30 (12.7) | 106.1 (18.0) | 110.1 (19.7) |
| ≥8.0 | 2586 (26.3) | 110.1 (15.4) | 113.4 (16.6) | 2 (0.9) | 106.0 (5.7) | 118.0 (18.4) |
| Lir ear trend † | <0.001 | <0.001 | <0.001 | <0.001 | ||
| Gestational age in weeks: | ||||||
| 33 to 37 | 599 (6.1) | 104.7 (15.6) | 108.5 (17.5) | 89 (37.7) | 96.6 (15.4) | 100.1 (15.7) |
| 38 to 39 | 1929 (19.6) | 107.6 (15.5) | 112.0 (16.6) | 82 (34.8) | 104.0 (14.2) | 108.5 (17.1) |
| 40 to 41 | 5179 (52.7) | 109.1 (15.6) | 112.8 (16.7) | 56 (23.7) | 102.8 (16.5) | 106.5 (20.1) |
| 42+ | 2125 (21.6) | 107.8 (16.0) | 111.5 (17.0) | 9. (3.8) | 115.1 (16.7) | 115.3 (11.6) |
| Linear trend † | <0.001 | <0.001 | <0.001 | <0.001 | ||
| Birth weight for gestational age (z score): | ||||||
| ≤0.61 | 2380 (24.2) | 106.0 (15.6) | 110.0 (16.5) | 144 (61.0) | 102.0 (15.5) | 104.8 (17.7) |
| -0.61 to -0.04 | 2470 (25.1) | 108.1 (15.9) | 111.8 (16.9) | 53 (22.5) | 101.5 (16.6) | 108.1 (16.1) |
| -0.05 to 0.63 | 2518 (25.6) | 108.9 (15.6) | 113.1 (17.1) | 32 (13.6) | 100.1 (15.9) | 102.6 (19.6) |
| ≥0.64 | 2464 (25.0) | 109.9 (15.4) | 113.4 (16.6) | 7 (3.0) | 91.9 (16.8) | 99.1 (17.1) |
| Linear trend † | <0.001 | <0.001 | 0.18 | 0.57 | ||
| Maternal age at delivery, in years: | ||||||
| 15-24 | 3496 (35.6) | 106.1 (15.1) | 110.5 (16.1) | 51 (21.6) | 101.5 (15.2) | 102.9 (17.0) |
| 25-29 | 3111 (31.6) | 109.2 (15.8) | 112.6 (17.1) | 78 (33.1) | 101.1 (14.5) | 106.5 (16.5) |
| 30-34 | 2089 (21.3) | 109.8 (15.9) | 113.5 (17.1) | 72 (30.5) | 100.4 (16.9) | 105.1 (19.4) |
| ≥35 | 1136 (11.6) | 109.7 (15.7) | 113.1 (17.4) | 35 (14.8) | 103.7 (17.6) | 105.3 (17.4) |
| Linear trend † | <0.001 | <0.001 | 0.80 | 0.51 | ||
| No of older siblings‡: | ||||||
| 0 | 3826 (38.9) | 109.3 (15.6) | 115.1 (16.2) | 47 (19.9) | 103.3 (15.4) | 109.9 (15.1) |
| 1 | 3090 (31.4) | 109.7 (15.6) | 113.1 (16.8) | 101 (42.8) | 101.8 (14.0) | 107.0 (16.7) |
| 2 | 1600 (16.3) | 105.8 (15.1) | 108.9 (16.6) | 40 (17.0) | 99.9 (18.8) | 99.4 (21.0) |
| ≥3 | 1316 (13.4) | 102.3 (14.9) | 104.8 (16.1) | 48 (20.3) | 99.8 (17.5) | 102.0 (16.9) |
| Linear trend † | <0.001 | <0.001 | 0.23 | <0.01 | ||
| Father's social class at birth: | ||||||
| I | 196 (2.0) | 123.1 (12.4) | 126.9 (15.5) | 8 (3.4) | 112.0 (20.4) | 116.5 (24.5) |
| II | 693 (7.1) | 118.7 (14.6) | 122.8 (16.4) | 10 (4.2) | 116.8 (18.0) | 107.6 (17.6) |
| III | 5620 (57.2) | 109.6 (15.3) | 113.5 (16.3) | 134 (56.8) | 101.8 (15.3) | 105.4 (16.9) |
| IV | 1388 (14.1) | 103.7 (14.1) | 107.8 (15.3) | 33 (14.0) | 103.9 (16.6) | 108.2 (14.7) |
| V | 1531 (15.6) | 101.9 (14.3) | 105.4 (15.8) | 41 (17.4) | 92.9 (14.0) | 95.8 (18.5) |
| Dead or unemployed | 404 (94.1) | 103.6 (15.4) | 107.7 (17.2) | 10 (4.2) | 107.3 (8.0) | 118.1 (10.1) |
| Linear trend † | <0.001 | <0.001 | 0.09 | 0.53 | ||
†
P value for the test for heterogeneity or linear trend in mean IQ values across the categories of each variable (excluding missing).