Table 1

Cognitive test scores in extremely preterm (EP) participants and term-born controls by age of assessment, overall and stratified by sex, maternal education, neonatal brain injury and gestational age

Age 2.5 yearsAge 6 yearsAge 11 yearsAge 19 years
EP (n=283)EP (n=241)Control (n=160)EP (n=219)Control (n=153)EP (n=127)Control (n=64)
Cognitive test*BSID-II MDIK-ABC MPCK-ABC MPCWASI-II FSIQ
Age at assessment†
 Mean (SD)2.5 (0.1)6.3 (0.5)6.1 (0.5)11.0 (0.4)11.0 (0.6)19.3 (0.6)19.2 (0.5)
 Median (range)2.5 (2.3–3.3)6.3 (5.2–7.3)6.2 (5.1–7.2)10.9 (10.2–12.2)10.9 (9.8–12.3)19.3 (18.4–20.5)19.2 (18.1–20.1)
Cognitive test score‡
 Mean (95% CI)78.7 (76.7 to 80.7)81.6 (79.0 to 84.2)105.7 (103.9 to 107.6)83.4 (80.8 to 85.9)104.1 (102.3 to 105.8)85.7 (82.7 to 88.8)103.9 (101.4 to 106.4)
Mean difference (95% CI)−24.2 (−27.7 to −20.6)−20.7 (−24.1 to −17.3)−18.1 (−22.8 to −13.5)
Infant sex
 Female
  n (%)148 (52.3)119 (49.4)89 (55.6)118 (53.9)89 (58.2)68 (53.5)39 (60.9)
  Mean (95% CI)82.4 (80.0 to 84.9)87.2 (83.9 to 90.4)105.5 (103.2 to 107.8)87.6 (84.5 to 90.6)104.2 (102.2 to 106.3)88.6 (84.9 to 92.3)103.6 (100.7 to 106.5)
 Male
  n (%)135 (47.7)122 (50.6)71 (44.4)101 (46.1)64 (41.8)59 (46.5)25 (39.1)
  Mean (95% CI)74.6 (71.7 to 77.6)76.1 (72.3 to 79.9)106.0 (103.0 to 109.0)78.5 (74.4 to 82.6)103.8 (100.8 to 106.9)82.4 (77.6 to 87.2)104.4 (99.7 to 109.0)
 Mean difference (95% CI)−7.8 (−11.6 to −4.0)−11.1 (−16.2 to −6.1)0.5 (−3.3 to 4.2)−9.1 (−14.1 to −4.0)−0.4 (−4.0 to 3.2)−6.2 (−12.2 to −0.2)0.8 (−4.5 to 6.1)
Maternal education
 O’ level or below
  n (%)174 (61.5)146 (60.6)82 (51.3)131 (59.8)87 (56.9)64 (50.4)32 (50.0)
  Mean (95% CI)77.9 (75.4 to 80.4)80.6 (77.4 to 83.8)104.7 (102.1 to 107.2)82.7 (79.4 to 86.1)102.6 (100.3 to 104.8)85.6 (81.7 to 89.4)103.4 (99.4 to 107.3)
 A’ level or above
  n (%)95 (33.6)83 (34.4)59 (36.9)77 (35.2)58 (37.9)60 (47.2)31 (48.4)
  Mean (95% CI)81.1 (77.9 to 84.3)83.8 (79.0 to 88.7)108.7 (105.6 to 111.7)86.3 (82.1 to 90.6)106.1 (103.0 to 109.1)87.3 (82.7 to 91.8)104.3 (101.1 to 107.5)
 Mean difference (95% CI)3.2 (−1.0 to 7.4)3.2 (−2.4 to 8.9)4.0 (−0.01 to 8.0)3.6 (−1.8 to 9.0)3.5 (−0.2 to 7.2)1.7 (−4.3 to 7.7)0.9 (−4.3 to 6.2)
Neonatal brain injury
 No/mild brain injury
  n (%)220 (77.7)187 (77.6)160 (100)169 (77.2)153 (100)104 (81.9)164 (100)
  Mean (95% CI)80.5 (78.3 to 82.6)85.2 (82.7 to 87.7)86.1 (83.6 to 88.5)87.2 (84.2 to 90.2)
 Moderate/severe brain injury
  n (%)63 (22.3)54 (22.4)0 (0.0)49 (22.4)0 (0.0)22 (17.3)0 (0.0)
  Mean (95% CI)72.7 (68.1 to 77.3)68.9 (62.0 to 75.8)73.8 (66.5 to 81.0)78.4 (68.8 to 88.1)
 Mean difference (95% CI)−7.8 (−12.4 to −3.12)−16.3 (−22.3 to 10.4)−12.3 (−18.3 to −6.3)−8.7 (−16.7 to −0.8)
Gestational age at delivery
 25 weeks
  n (%)167 (59.0)144 (59.8)0 (0.0)126 (57.5)0 (0.0)75 (59.1)0 (0.0)
  Mean (95% CI)79.8 (77.5 to 82.2)84.3 (81.2 to 87.5)86.1 (83.3 to 88.9)87.6 (84.2 to 91.0)
 <25 weeks
  n (%)116 (41.0)97 (40.2)0 (0.0)93 (42.5)0 (0.0)52 (40.9)0 (0.0)
  Mean (95% CI)77.1 (73.8 to 80.4)77.4 (73.1 to 81.8)79.7 (75.1 to 84.4)83.1 (77.7 to 88.6)
 Mean difference (95% CI)−2.7 (−6.7 to 1.3)−6.9 (−12.2 to 1.6)−6.3 (−11.5 to 1.2)−4.5 (−10.6 to 1.7)
  • *BSID-II MDI, Bayley Scales of Infant Development -Second Edition Mental Development Index; K-ABC MPC, Kaufman Assessment Battery for Children  mental processing composite; WASI-II FSIQ, Wechsler Abbreviated Scale of Intelligence-Second Edition – Full Scale IQ.

  • †Corrected age at year 2.5 and chronological age in years 6, 11 and 19.

  • ‡Values were multiply imputed within a clinically specified range for EP participants seen at assessment but not tested with planned cognitive test: n=52 (year 2.5), n=41 (year 6), n=18 (year 11), n=3 (year 19). Values were multiply imputed for one control in year 6.