- A prospective study was made in 153 patients with a second stage of labour lasting 1 to 120 min. Maternal venous, umbilical artery and vein blood were analysed for acid-base variables and lactate levels. Blood was obtained immediately at delivery. Fetal heart rate monitoring data and blood test results were related to the duration of the second stage of labour and clinical outcome. In primiparae and multiparae there was a steady rise in umbilical artery lactate levels, with duration of second stage of labour, P less than 0.025 and P = 0.023, respectively. The median umbilical artery pH decreased significantly from 7.31 in patients with a second stage of labour that lasted less than or equal to 15 min to a pH of 7.25 in patients with a second stage of labour that lasted more than 30 min. Changes in PO2 and PCO2 were not statistically significant.