Study demographics
| Extremely preterm (≤27+6) | Very preterm (28+0–32+0) | Term (≥37+0) | |
| Infant characteristics | |||
| Number of participants* | 16 | 42 | 45 |
| Birth gestation (weeks) | 26.4 (25.2–27.2) | 30.8 (29.5–31.3) | 40.1 (39.2–41.0) |
| Male, n (%) | 12 (75) | 28 (67) | 21 (47) |
| Female, n (%) | 4 (25) | 14 (33) | 24 (53) |
| Birth weight (g) | 909±219 | 1436±388 | 3556±437 |
| Birthweight z-score† | 0.3±0.7 | −0.2±1.3 | 0.5±1 |
| Singleton, n (%) | 12 (75) | 30 (71) | 45 (100) |
| Twin, n (%) | 4 (25) | 12 (29) | 0 (0) |
| Sampling characteristics | |||
| Chronological age at diurnal samples (weeks) | 19.9 (17.3–23.5) | 18.4 (16.9–21.1) | 17.1 (16.3–18.9) |
| Corrected age at diurnal samples (weeks) | 5.4 (4.0–8.9) | 8.9 (7.9–11.2) | |
| Chronological age at vaccine (weeks) | 20 (16.8–25.6) | 18.8 (17.3–20.8) | 17.1 (16.4–18.7) |
| Corrected age at vaccine (weeks) | 5.6 (3.7–11.8) | 9.0 (8.1–11.8) | |
| Time of day at morning sample | 08:45 (08:08–09:48) | 08:42 (08:06–09:20) | 08:23 (07:50–09:05) |
| Time of day at evening sample | 20:15 (19:40–21:11) | 20:19 (19:46–21:19) | 19:52 (19:34–20:30) |
| Time of day at vaccine | 11:37 (11:14–13:35) | 11:31 (10:34–13:36) | 11:03 (10:05–13:21) |
Continuous data are presented as mean±SD if normally distributed and median (IQR) if skewed.
*Data were unavailable for 2 participants around vaccination and 13 for diurnal assessments due to missed or insufficient volume collections.
†Birthweight z-scores were calculated as per the International Fetal and Newborn Growth Consortium for the 21st Century (INTERGROWTH) standards.