Melatonin in the maternal and umbilical circulations during human parturition.

The concentration of melatonin in maternal peripheral plasma was measured during late pregnancy, term and pre-term labour. There was a small increase in the mean concentration of melatonin during labour which was significant in term labour. Umbilical arterial and venous plasma, whether obtained at term, after spontaneous vaginal delivery or at Caesarean section, contained significantly greater concentrations of melatonin than maternal plasma. A significant arterio-venous difference was demonstrated for both groups of umbilical samples with raised venous levels after spontaneous vaginal delivery but higher arterial levels at Caesarean section.