Association of gestational day with antenatal management and the mortality and respiratory outcomes of extremely preterm infants.

Objective: Perinatal epidemiological studies and outcomes are often reported on gestational week thresholds. This study aims to quantify and investigate the association of each gestational day at birth on antenatal management, mortality and respiratory outcomes of extremely preterm infants.

Design: Retrospective cohort study using National Neonatal Research Database.

Setting: England and Wales.

Patients: 26 098 infants born <28 weeks of gestational age (GA) and admitted to neonatal units from 2010 to 2020.

Interventions: Antenatal care and outcome measures for each gestational day were described with 95% CI determined using Agresti-Coull method. χ test for trend assessed the trends across gestational day. Analysis of means assessed if outcome on each gestational day differed from the overall outcome for that gestational week.

Main Outcome Measures: Mortality and respiratory disease.

Results: Neonatal admissions peaked at the start of each gestational week. Caesarean section was the most common birth mode from 26 to 26 weeks GA. Mortality and severe respiratory morbidity decreased with each day of gestation within the gestational week threshold (p<0.01). Mortality at the beginning and end of each gestational week differed from the overall mortality for that gestational week (p=0.03 to <0.001) in infants <27 weeks GA. Mortality was higher in infants <26 weeks GA born to mothers without complete antenatal corticosteroid course or born in centres without neonatal intensive care units.

Conclusions: Each day of gestation is important for extremely preterm infant outcomes. Perinatal decision-making, counselling and reporting should avoid broad gestational weeks and include day of gestation.