Background In 2006, the Dutch guideline for active treatment of extremely preterm neonates advised to lower the gestational age threshold for active intervention from 26 0/7 to 25 0/7 weeks gestation.

Objective To evaluate the association between the guideline modification and early neonatal outcome.

Design National cohort study, using prospectively collected data from The Netherlands Perinatal Registry.

Patients The study population consisted of 9713 infants with a gestational age between 24 0/7 and 29 6/7 weeks, born between 2000 and 2011. Three gestational age subgroups were analysed: 24 0/7 to 24 6/7 weeks (n=269), 25 0/7 to 25 6/7 weeks (n=852) and 26 0/7 to 29 6/7 weeks (n=8592).

Main outcome measures Neonatal intensive care unit (NICU) admission, live births, neonatal in-hospital mortality, morbidity and favourable outcome (no mortality or morbidity) before (2000–2005; period 1) and after (2007–2011; period 2) introduction of the modified guideline, using χ² tests and univariable and multivariable logistic regression analyses.

Results In the second period, the proportion of live births and NICU admissions increased and the proportion of neonatal and in-hospital mortality decreased significantly in all subgroups. Morbidity in surviving infants of 25 weeks increased significantly, although the association between guideline modification and morbidity became non-significant after case-mix adjustment. Overall, favourable outcome did not change significantly after guideline modification in all subgroups when adjusted for variation in case-mix.

Conclusions Overall, the trend in mortality gradually declined at all gestational ages, starting before 2006. This suggests that the guideline modification was a formalisation of already existing daily practice.