A previous study published in 2008 by Wiberg et al demonstrated increasing umbilical cord blood lactate at delivery by gestational age in vigorous offspring (n = 10 169, gestational age 24-43 weeks). Based on these results the authors concluded that gestational age-independent umbilical cord lactate cut-off could give false-negative or false-positive diagnosis of lacticemia. To our knowledge, these findings have not been incorporated into clinical interpretations in delivery units. To perform an external validity study for the findings by Wiberg et al, we analyzed umbilical cord blood lactate levels according to gestational age in a post-date delivery study population at our large, tertiary obstetric unit. The parallel finding of our study to that of Wiberg et al highlights the importance of using available gestational age dependent reference ranges (eg as presented in Wiberg's publication), when interpreting umbilical cord blood lactate levels for fetal wellbeing. © 2020 The Authors. Acta Obstetricia et Gynecologica Scandinavica published by John Wiley & Sons Ltd on behalf of Nordic Federation of Societies of Obstetrics and Gynecology (NFOG).