On behalf of Australian and New Zealand Intensive Care Society Paediatric Study Group (ANZICS-PSG) & Australian and New Zealand Paediatric Intensive Care Registry (ANZPICR) Introduction: Population level data shows that the gestational age at birth is decreasing in various countries. Gestational age is an important and potentially modifiable risk factor and is particularly relevant in the neonate with congenital heart disease (CHD) where a high proportion of births occur before reaching full term. We studied the relationship between gestation & hospital mortality in a multi-center cohort of neonates undergoing cardiac surgery in Australia and New Zealand. Methods: A total of 2,267 neonates with CHD who underwent cardiac surgery during 2007 - 2016 were included. For the primary outcome of hospital mortality, we determined the best fitting first or second order fractional polynomial to describe its association with gestational age using multivariable logistic regression. The analysis was controlled for age, sex, indigenous status, centre and risk adjusted congenital heart surgery (RACHS-1) score. Results: The median (IQR) gestation at birth was 39 (37.6 - 40) weeks and hospital mortality following cardiac surgery was 7.4% (169 out of 2267). For each week increase in gestation, there was a 20% reduction in in-hospital mortality [adjusted odds ratio 0.80, 95%CI 0.74, 0.87, p<0.001]. On average, birth at 37 weeks was associated with mortality that was approximately 4 percentage points higher (80% increase) than those born at 40 weeks (figure). Among survivors, the mean intensive care length of stay decreased for each week increase in gestation [-4.7% per week, 95%CI -6.5%, -2.7%, p<0.001]. Conclusion: Increasing gestation, even within the ranges of term gestation, is associated with a considerable decrease in the risk of in-hospital mortality following neonatal cardiac surgery. This information is particularly relevant in the current context of a high number of planned births before reaching full term in the neonate with CHD, and underscores the urgent need for development of standard guidelines for timing of delivery. [ABSTRACT FROM AUTHOR]