Ten patients, hospitalized because of severe asymmetrical fetal growth retardation before 32 weeks gestation, underwent ultrasonically guided percutaneous umbilical blood sampling because of concern over the fetal heart rate trace. In eight patients the fetus was judged to be acidotic and they were delivered immediately by caesarean section. In two patients the fetus was not considered to be acidotic and the pregnancy was allowed to go on. In those babies that were delivered the pH blood collected from the umbilical vein at the time of delivery was compared to the antenatal sample. There were no significant differences. The technique of antenatal umbilical vein blood sampling is readily learned and by assessment of fetal acid base status a more precise diagnosis can be made leading to appropriately planned management.