Time-resolved spectroscopy (TRS-20) measures tissue oxygen saturation (%) by evaluating the absolute concentrations of oxygenated, deoxygenated and total haemoglobin based on measurement of the transit time of individual photons through a tissue of interest. We measured tissue oxygen saturation in the prefrontal lobes of the brain by TRS-20 in eighteen pregnant women during caesarean section. In a case of placenta previa, massive bleeding immediately decreased cerebral oxygen saturation from 67·2% to 54·2%, but did not alter peripheral tissue oxygenation as measured by pulse oximetry. Four cases of pre-eclampsia revealed chronic changes in elevated base levels of cerebral oxygen saturation, though peripheral oxygen saturation was similar to that in normotensive pregnant women. Average cerebral oxygen saturation in the cases of pre-eclampsia before the introduction of anaesthesia was 73·6 ± 4·4 (SD)% (n = 4), significantly higher than in normotensive pregnant women, 67·2 ± 4·3% (n = 13, P<0·05). Z-scores of cerebral oxygen saturation prior to anaesthesia positively correlated with those of systolic or diastolic blood pressure. TRS-20 could detect acute as well as chronic changes in brain oxygen saturation in response to pregnancy-associated complications.