Lactate values during labour and their correlation with maternal and fetal outcome.

Abstract: This study investigates the relationship between lactate levels - produced during myometrial contraction and transient hypoxia - and maternal and fetal outcomes. Uterine contractions during labour are regulated by hormonal and metabolic processes, including oxytocin release, catecholamine modulation, and the hypoxia-induced force increase (HIFI) mechanism. In a cohort of 303 women receiving neuraxial analgesia, lactate was measured at three time points: at the diagnosis of the active phase of the first stage of labour (T0), at the beginning of active pushing (T1), and at delivery (T2). Lactate values increased progressively from T0 to T2, with median values exceeding 2 mmol/L at T1 and T2 (T1 = 2.84, SD = 1.32, and T2 = 3.80, SD = 1.56). However, no significant correlation was found between lactate levels and postpartum haemorrhage; both blood loss and the interaction term (time and blood loss) were not statistically significant (P = 0.574 and P = 0.211, respectively). Lactate levels did not significantly correlate with the mode of onset of labour or neonatal umbilical cord pH alterations. Instead, multiparous women exhibited lower lactate levels at T1 and T2 (P < 0.001) compared with nulliparous women, likely due to a more efficient myometrial response and metabolic adaptation. These findings suggest that lactate, rather than solely indicating hypoxia, could play a physiological role in labour and should not be regarded exclusively as a marker of distress. Understanding its role could refine the clinical interpretation of maternal metabolic stress and improve labour management strategies. Lay summary: Many women die after childbirth because of severe bleeding. Bleeding occurs mostly when the uterus is unable to contract properly. Identifying a blood biomarker that defines this risk during labour would be important. We considered lactate a possible biomarker. Lactate is produced by cells when oxygen levels drop, as may happen during labour when the uterus contracts rhythmically for many hours. We measured lactate levels in 303 women who had a type of pain relief known as neuraxial analgesia, where a local anaesthetic is given into the spine. We found that lactate levels increased during labour and differed depending on whether a woman had given birth before or not. However, lactate levels were not linked to postpartum haemorrhage, nor did they affect the baby's well-being. Our findings suggest that lactate is not always a sign of distress but rather a potential normal factor to enhance the understanding and management of labour.