In pregnancy the feto-placental unit is the major source of activin A. However, the role(s) of activin A in late pregnancy remain uncertain and controversial. In particular, whether activin A levels alter in association with labour is unclear. In a cross-sectional cohort study, maternal serum samples were collected from women at term prior to elective Caesarean section (n=11), during labour prior to a spontaneous vaginal delivery (n=31), an instrumental vaginal delivery (n=16) or an emergent Caesarean section (n=7). Umbilical artery blood samples were collected from 75 pregnancies, after an elective Caesarean section (n=9), a normal vaginal delivery (n=37), an instrumental vaginal delivery (n=15) or an emergent Caesarean section (n=14). Levels of activin A were measured and compared according to modes of delivery. Maternal, but not foetal, serum activin A was increased significantly in women who were delivered by an intrapartum Caesarean section compared to other modes of delivery. Foetal, but not maternal, serum activin A was significantly correlated with umbilical artery pH. Maternal serum activin A is increased in women undergoing an intrapartum Caesarean section compared to either a vaginal delivery or an elective Caesarean section. The mechanism(s) underlying this observation are not clear.