Follow-up results of all 554 infants with follow-up data available born <27 weeks’ gestational age (GA) who reached 2 years’ corrected age (CA) in 2018–2020, separately for infants born at 24, 25 and 26 weeks’ gestation. All follow-up data are presented as N (%), mean (SD) or median (IQR). Impairment rates are presented as % (95% CI). The follow-up programme included a physical and neurological examination and assessment of mental and psychomotor development with the Dutch version of the Bayley Scales of Infant and Toddler Development (Bayley-III-NL, mean 100 (SD 15)). Parents are asked to fill out the Child Behavioral Checklist (CBCL) to rate internalising, externalising and total behaviour problems (mean 50 (SD 10), higher scores indicate more behavioural problems). Neurological examination was performed to determine cerebral palsy (CP), which was graded using the five levels defined in the Gross Motor Function Classification System (GMFCS), from 1 for minimal impairment to 5 for severe impairment with dependence on caretakers for most daily activities. A combination of medical history and results of the assessment was used to rate hearing and vision status. Outcome measures were compared between GA groups using a one-way analysis of variance or Mann-Whitney U-test for continuous variables, depending on distribution, and a χ2 test for categorical variables. A value of p<0.05 is considered statistically significant
| Total | 24 weeks | 25 weeks | 26 weeks | P value | |
| Follow-up data available | n=554 | n=98 | n=178 | n=278 | |
| Bayley-III-NL cognitive score, N (%) evaluated | 497 (89.7) | 87 (88.8) | 162 (91.0) | 248 (89.2) | |
| Bayley-III-NL cognitive score mean (SD) | 96 (14) | 94 (12) | 95 (15) | 97 (13) | 0.107 |
| Bayley-III-NL cognitive score median (IQR) | 96 (87–105) | 96 (87–101) | 96 (87–101) | 96 (88–105) | 0.135 |
| Bayley-III-NL cognitive score 85 or greater | 404 (81.3) | 70 (80.5) | 124 (76.5) | 210 (84.7) | 0.073 |
| Bayley-III-NL cognitive score 70–84 | 77 (15.5) | 12 (13.8) | 30 (18.5) | 35 (14.1) | |
| Bayley-III-NL cognitive score <70 | 16 (3.2) | 5 (5.7) | 8 (4.9) | 3 (1.2) | |
| Bayley-III-NL motor score, N (%) evaluated | 437 (78.9) | 80 (81.6) | 135 (75.8) | 222 (79.9) | |
| Bayley-III-NL Bayley motor score mean (SD) | 95 (14) | 92 (15) | 97 (14) | 95 (14) | 0.037* |
| Bayley-III-NL Bayley motor score median (IQR) | 95 (84–105) | 94 (83–101) | 98 (87–107) | 95 (87–106) | 0.053 |
| Bayley-III-NL Bayley motor score 85 or greater | 326 (74.6) | 51 (63.8) | 107 (79.3) | 168 (75.7) | 0.093 |
| Bayley-III-NL Bayley motor score 70–84 | 94 (21.5) | 23 (28.8) | 25 (18.5) | 46 (20.7) | |
| Bayley-III-NL Bayley motor score <70 | 17 (3.9) | 6 (7.5) | 3 (2.2) | 8 (3.6) | |
| Vision, N (%) evaluated | 533 (96.2) | 95 (96.9) | 170 (95.5) | 268 (96.4) | |
| Normal | 450 (84.4) | 79 (83.2) | 140 (82.4) | 231 (86.1) | 0.175 |
| Mild visual problems including squints or refractive errors | 67 (12.6) | 16 (16.8) | 23 (13.5) | 28 (10.4) | |
| Functionally impaired vision or blindness | 16 (3.0) | 0 (0.0) | 7 (4.1) | 9 (3.4) | |
| Hearing, N (%) evaluated | 529 (95.5) | 94 (95.9) | 171 (96.1) | 264 (95.0) | |
| Normal | 508 (96.0) | 92 (97.9) | 162 (94.7) | 254 (96.2) | 0.356 |
| Mild hearing loss, not sufficient to require aids | 16 (3.0) | 2 (2.1) | 8 (4.5) | 6 (2.3) | |
| Hearing loss requiring aids or severe sensorineural hearing loss despite aids | 5 (0.9) | 0 (0.0) | 1 (0.6) | 4 (1.5) | |
| Neurological exam, N (%) evaluated | 530 (95.7) | 95 (96.9) | 170 (95.5) | 265 (95.3) | |
| Normal | 440 (83.0) | 72 (75.8) | 142 (83.5) | 226 (85.3) | 0.116 |
| Mildly abnormal (eg, posture, coordination or tone dysregulation disorders) | 67 (12.6) | 17 (17.9) | 24 (14.1) | 26 (9.8) | |
| CP | 23 (4.3) | 6 (6.3) | 4 (2.4) | 13 (4.9) | |
| Classification of CP | |||||
| Spastic unilateral | 8 (34.8) | 2 (33.3) | 0 (0.0) | 6 (46.2) | |
| Spastic bilateral | 8 (34.8) | 2 (33.3) | 1 (25.0) | 5 (38.5) | |
| Atactic | 2 (8.7) | 2 (33.) | 0 (0.0) | 0 (0.0) | |
| Dyskinetic | 1 (4.3) | 0 (0.0) | 0 (0.0) | 1 (7.7) | |
| Unclassifiable | 4 (17.4) | 0 (0.0) | 3 (75.0) | 1 (7.7) | |
| Classification according to GMFCS | |||||
| GMFCS I | 7 (30.4) | 3 (50.0) | 0 (0.0) | 4 (30.8) | |
| GMFCS II | 5 (21.7) | 1 (16.7) | 1 (25.0) | 3 (23.1) | |
| GMFCS III | 3 (13.0) | 1 (16.7) | 0 (0.0) | 2 (15.4) | |
| GMFCS IV | 2 (8.7) | 0 (0.0) | 0 (0.0) | 2 (15.4) | |
| Unknown | 6 (26.1) | 1 (16.7) | 3 (75.0) | 2 (15.4) | |
| CBCL Total T-score, N (%) evaluated | 397 (71.7) | 70 (71.4) | 130 (73.0) | 197 (70.9) | |
| CBCL Total T-score mean (SD) | 48 (10) | 49 (9) | 47 (10) | 48 (10) | 0.717 |
| CBCL Total T-score median (IQR) | 47 (40–55) | 47 (43–54) | 47 (39–55) | 48 (40–56) | 0.677 |
| CBCL Total T-score 60 or lower | 358 (90.2) | 64 (91.4) | 118 (90.8) | 176 (89.3) | 0.605 |
| CBCL Total T-score 61–70 | 34 (8.6) | 4 (5.7) | 11 (8.5) | 19 (9.6) | |
| CBCL Total T-score >70 | 5 (1.3) | 2 (2.9) | 1 (0.8) | 2 (1.0) | |
| CBCL Internalising T-score, N (%) evaluated | 398 (71.8) | 71 (72.4) | 130 (73.0) | 197 (70.9) | |
| CBCL internalising T-score mean (SD) | 47 (11) | 48 (9) | 46 (11) | 47 (11) | 0.763 |
| CBCL internalising T-score median (IQR) | 45 (41–55) | 47 (41–55) | 45 (37–55) | 45 (37–55) | 0.712 |
| CBCL internalising T-score 60 or lower | 359 (90.2) | 66 (93.0) | 118 (90.8) | 175 (88.8) | 0.755 |
| CBCL internalising T-score 61–70 | 33 (8.3) | 5 (7.0) | 10 (7.7) | 18 (9.1) | |
| CBCL internalising T-score >70 | 6 (1.5) | 0 (0.0) | 2 (1.5) | 4 (2.0) | |
| CBCL Externalising T-score, N (%) evaluated | 398 (71.8) | 71 (72.4) | 130 (78.0) | 197 (70.9) | |
| CBCL Externalising T-score mean (SD) | 49 (10) | 50 (9) | 49 (10) | 49 (10) | 0.925 |
| CBCL Externalising T-score median (IQR) | 50 (42–56) | 48 (44–56) | 48 (42–56) | 50 (42–56) | 0.869 |
| CBCL Externalising T-score 60 or lower | 356 (89.4) | 66 (93.0) | 114 (87.7) | 176 (89.3) | 0.828 |
| CBCL Externalising T-score 61–70 | 36 (9.0) | 4 (5.6) | 14 (10.8) | 18 (9.1) | |
| CBCL Externalising T-score >70 | 6 (1.5) | 1 (1.4) | 2 (1.5) | 3 (1.5) | |
| Impairment, including Bayley-III-NL cognitive score, vision, hearing and neurological exam, N (%) evaluated | 554 (100) | 98 (100) | 178 (100) | 278 (100) | |
| None | 61.9% (58–66) | 55.1% (45–65) | 56.7% (49–64) | 67.6% (62–73) | 0.072 |
| Mild | 29.1% (25–33) | 35.7% (27–46) | 33.7% (27–41) | 23.8% (19–29) | |
| Moderate-to-severe | 9.0% (7–12) | 9.2% (5–17) | 9.6% (6–15) | 8.6% (6–13) | |
| Impairment, including the above and also Bayley-III-NL motor score, N (%) evaluated | 464 (83.8) | 83 (84.7) | 146 (82.0) | 235 (84.5) | |
| None | 53.0% (48–58) | 43.4% (33–54) | 51.4% (43–59) | 57.4% (51–64) | 0.225 |
| Mild | 34.3% (30–39) | 43.4% (33–54) | 35.6% (28–44) | 30.2% (25–36) | |
| Moderate-to-severe | 12.7% (10–16) | 13.2% (8–22) | 13.0% (8–19) | 12.3% (9–17) | |
| Impairment, including the above and also CBCL T-scores, N (%) evaluated | 364 (65.7) | 69 (70.4) | 118 (66.3) | 177 (63.7) | |
| None | 45.6% (41–51) | 40.6% (30–52) | 40.7% (32–50) | 50.9% (44–58) | 0.249 |
| Mild | 35.7% (31–41) | 42.0% (31–54) | 40.7% (32–50) | 29.9% (24–37) | |
| Moderate-to-severe | 18.7% (15–23) | 17.4% (10–28) | 18.6% (13–27) | 19.2% (14–26) |
*Pairwise analysis showed a significant difference in Bayley-III-NL motor score between 24 weeks’ and 25 weeks’ gestation (p=0.014).