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To evaluate the association between lactate concentrations in fetal blood samples and the different advanced labour stages. Methods: Eighteen-month prospective population-based clinical study of 187 singleton pregnant women in labour who were monitored by fetal blood sampling (FBS) because of non-reassuring intrapartum CTG results at Kuopio University Hospital, Finland. Peripheral lactate concentration and pH were analysed at different stages of labour and in umbilical arterial samples immediately after delivery. Results: FBS samples (N = 350) were obtained from 5.4 % of all women in labour during the study period, and 48 % had spontaneous delivery, 27 % had vacuum-assisted vaginal delivery, and 25 % had nonelective Caesarean delivery. FBS lactate levels increased 4-11% with every 1-2 cm of cervical dilation and 18 % from early labour to fully dilated cervix. In 42 women with at least two FBSs, lactate levels increased significantly from the early I stage of labour and up to a fully dilated cervix. Lactate values were significantly higher in umbilical arterial samples compared to FBSs. Sensitivity of the highest FBS lactate values for the detection of birth asphyxia were considerably low varying between 42.9-57.1%. Conclusion: FBS lactate levels were related to the stage of labour during vaginal delivery attempt. Wide range of lactate levels during labour complicates its use as a predictor of birth asphyxia.