Neonatal acid-base equilibrium at birth: an analysis of the ante and intra-partum determinants and their relevance on adverse perinatal outcomes.

Objective: To determine antepartum and intrapartum risk factors authentically linked to the onset of neonatal metabolic acidosis, with possible hypoxic-ischemic encephalopathy (HIE) development. Methods: Monocentric retrospective cohort study conducted from 2016 to 2020. All term neonates born from single pregnancies to mothers over 18 years old, with recorded arterial blood gas analysis at birth and without chromosomal abnormalities or structural malformations, were included in the study. Based on the outcome of the umbilical artery blood gas analysis at birth patients were divided into the metabolic acidosis group, defined by a pH of ≤ 7.0 and/or a base excess of ≤ -12 mmol/L, and the control group comprising the remaining others. The two groups were compared for each risk factor using descriptive analysis, and those found to be statistically significant were further investigated using logistic regression. Results: Of the 13023 deliveries included in the study, patients were divided into the metabolic acidosis group with umbilical artery pH ≤ 7.0 and/or base excess ≤ -12 mmol/L (n=395) and the control group with normal pH and base excess (n=12628). Intrapartum risk factors significantly associated with metabolic acidosis included maternal pyrexia (OR 2.506, CI 1.140 - 5.510), sentinel events such cord prolapse, placental abruption and ruptured uterus (OR 2.453, CI 1.609 – 3.740) and oxytocin administration. Although meconium-stained amniotic fluid showed significant association with metabolic acidosis in the descriptive analysis, its odd ratio was not found to be significant. Prolonged membrane rupture was significant only when lasting more than 24 hours, but surprisingly more common in the control group. Among the antepartum risk factors, significant differences were observed in the prevalence of hypertensive disorders of pregnancy (OR 1.971, CI 1.449 – 2.679), labor induction (OR 1.446, CI 1.171 – 1.785) and intrauterine growth restriction (IUGR) (OR 3.611, CI 1.384 – 9.421), together with some obstetric characteristics such nulliparity (OR 2.208, CI 1.748 – 2.788) and mode of delivery. Specifically, spontaneous vaginal delivery and both elective and emergency cesarian section were more common in the control, on the contrary operative vaginal delivery was more common in the metabolic acidosis group. Interestingly, chronic hypertension showed a trend towards significance (p = 0.0514), indicating a possible association with metabolic acidosis. Conclusions: The identified statistically significant intrapartum risk factors are more numerous than the antepartum ones. However, the latter include some of the most strongly associated risk factors for metabolic acidosis development, first among all IUGR. In conclusion, the findings of this study underscore the importance of identifying and managing both antepartum and intrapartum risk factors to mitigate the risk of metabolic acidosis and its severe neonatal outcomes.