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Objectives To address the practical problems of routine umbilical cord blood sampling, to determine the ranges for pH, pCO₂ and base deficit and to examine the relationships of these parameters between cord vessels.

Design An observational study of umbilical cord artery and vein blood gas results.

Setting A large district general hospital in the UK.

Subject One thousand nine hundred and forty‐two cord results from 2013 consecutive pregnancies of 34 weeks or more gestation, monitored by fetal scalp electrode during labour.

Results Only 1448 (74.6%) of the 1942 supposedly paired samples had validated pH and pCO₂data both from an artery and the vein; 54 (2.8%) had only one blood sample available, 90 (4.6%) had an error in the pH or pCO₂ of one vessel and in 350 (18%) pairs the differences between vessels indicated that they were not sampled from artery and vein as intended. Only 60 …