Table 1. Comparison of characteristics between participants with IQ measured at 8 or 15 years and those that did not have IQ measured1.
| IQ measured at 8 or 15 years (n = 7652) | IQ not measured (n = 6326) | Mean difference (95% CI), [P value]2 | |||||
| n with data | Mean (SD) or n (%) | n with data | Mean (SD) or n (%) | ||||
| Birth and perinatal characteristics | |||||||
| Gestational age at birth (wk) | 7652 | 39.4±1.9 | 6326 | 39.4±1.9 | −0.03 (−0.1, 0.03) [0.30] | ||
| Birth weight (kg) | 7562 | 3.41±0.55 | 6326 | 3.37±0.57 | −0.04 (−0.06, −0.03) [<0.001] | ||
| Birth length (cm) | 6049 | 50.6±2.5 | 4487 | 50.5±2.5 | −0.1 (−0.2, −0.0) [0.01] | ||
| Sex (Male) | 7652 | 3799 (50) | 6324 | 3421 (54) | [<0.001] | ||
| Singleton | 7652 | 7442 (97) | 6326 | 6175 (98) | [0.19] | ||
| Maternal age at birth (y) | 7652 | 29.0±4.6 | 6326 | 26.8±5.1 | −2.2 (−2.4, −2.1) [<0.001] | ||
| Parity | 7370 | 5559 | [<0.001] | ||||
| 0 | 3423 (46) | 2349 (42) | |||||
| 1 | 2633 (36) | 1906 (34) | |||||
| 2 | 974 (13) | 877 (16) | |||||
| 3 | 263 (4) | 277 (5) | |||||
| ≥4 | 77 (1) | 150 (3) | |||||
| Alcohol intake at 32 wk gestation | 4243 | 2648 | [<0.001] | ||||
| None | 2804 (66) | 1896 (72) | |||||
| 1–7 per week | 1179 (28) | 588 (22) | |||||
| >7 per week | 260 (6) | 164 (6) | |||||
| Maternal pre-pregnancy BMI | 6875 | 22.9±3.7 | 4659 | 22.9±4.0 | −0.01 (−0.2, 0.1) [0.89] | ||
| Maternal and family characteristics | |||||||
| Tobacco smoking | 7079 | 4355 | [<0.001] | ||||
| Never | 3918 (55) | 1886 (43) | |||||
| Quit | 2177 (31) | 1280 (29) | |||||
| Smoked during last trimester of pregnancy | 984 (14) | 1189 (27) | |||||
| Any breastfeeding | 7008 | 4133 | [<0.001] | ||||
| Never | 1318 (19) | 1438 (35) | |||||
| ≤1 month | 1104 (16) | 706 (17) | |||||
| 1 to <3 months | 1119 (16) | 648 (16) | |||||
| 3 to <6 months | 992 (14) | 435 (11) | |||||
| ≥6 months | 2475 (35) | 906 (22) | |||||
| White ethnicity | 7616 | 7277 (96) | 6058 | 5629 (93) | [<0.001] | ||
| Maternal marital status | 7460 | 5627 | [<0.001] | ||||
| First marriage | 5556 (75) | 3403 (61) | |||||
| Subsequent marriage/s | 478 (6) | 369 (7) | |||||
| Widowed/divorced/separated | 364 (5) | 418 (7) | |||||
| Never married | 1062 (14) | 1437 (26) | |||||
| Maternal education3 | 7356 | 5062 | [<0.001] | ||||
| None/CSE | 1000 (14) | 1504 (30) | |||||
| Vocation | 651 (9) | 573 (11) | |||||
| O level | 2590 (35) | 1706 (34) | |||||
| A level | 1953 (27) | 841 (17) | |||||
| Degree or higher | 1163 (16) | 438 (9) | |||||
| Maternal social class4 | 6353 | 3710 | [<0.001] | ||||
| I | 432 (7) | 159 (4) | |||||
| II | 2205 (35) | 964 (26) | |||||
| II (non-manual) | 2673 (42) | 1631 (44) | |||||
| III (manual) | 430 (7) | 359 (10) | |||||
| IV | 523 (8) | 469 (13) | |||||
| V | 90 (1) | 128 (3) | |||||
| Family income (£ per week) | 6088 | 2753 | [<0.001] | ||||
| <100 | 393 (6) | 377 (14) | |||||
| 100–199 | 946 (16) | 615 (22) | |||||
| 200–299 | 1757 (29) | 756 (27) | |||||
| 300–399 | 1383 (23) | 496 (18) | |||||
| ≥400 | 1609 (26) | 509 (18) | |||||
| HOME score5 | 7165 | 8.2±2.2 | 4307 | 7.8±2.3 | −0.3 (−0.4, −0.3) [<0.001] | ||
| Number of other children | 7112 | 4274 | [<0.001] | ||||
| 0 | 3212 (45) | 1713 (40) | |||||
| 1 | 2630 (37) | 1571 (37) | |||||
| 2 | 977 (14) | 698 (16) | |||||
| 3 | 232 (3) | 208 (5) | |||||
| ≥4 or more | 61 (1) | 84 (1) | |||||
Abbreviations; BMI, body mass index; CSE, Certificate of Secondary Education; IQ, intelligence quotient; WASI, Wechsler Abbreviated Scale of Intelligence; WISC, Wechsler Intelligence Scale for children
Statistical tests were used to compare characteristics of participants who had IQ measured at 8 or 15 years, against participants who had no IQ measurements. Continuous variables were compared by using independent t-test and categorical variables were compared by using χ2 tests.
Maternal education is reported as the highest completed level on five ordinal categories from Certificate of Secondary Education(CSE), Vocational training, O(ordinary)-level (taken by the top 25% of CSE at 15 years), A(advanced)-level (involving 2 years of study beyond O-level) and degree or higher. The CSE, O-levels and A-levels are completed at secondary school.
Social class was categorized according to maternal occupation during pregnancy, according to standard UK classifications of occupation, ranging from class I (highest), II, III-non-manual, III-manual, IV, and V (lowest)[17].
Stimulation in the home environment was measured by an adaptation of the HOME questionnaire at 18 months of age[18].