Reference values for umbilical cord arterial pH by mode of birth in preterm, term and postterm infants: a population-based cohort study

Umbilical cord arterial pH (UApH) is the most objective measurement of the metabolic condition of the infant at birth. However, studies have identified changes in UApH with advancing gestational age, as there is limited data examining UApH changes with different modes of birth. The objective was to establish reference values for UApH according to mode of birth and to investigate the influence of gestational age on changes in UApH across different modes of birth. A retrospective cohort study using data from Perinatal Revision South register, from seven maternity units in southern Sweden. Validated pH values from 103,115 infants were included. Cohort characteristics were reported collectively and for each mode of birth using numbers and percentages. Reference values were reported as 2.5th to 97.5th percentiles, with mean and median values. One-way ANOVA was used to investigate differences in UApH across different modes of birth. Simple and multiple linear regressions were performed associating gestational age with pH by mode of birth. Mode of birth in the study population consistent of vaginal non-instrumental (80,957/78.5%), vacuum-assisted (6,486/6.3%), forceps-assisted (429/0.4%), elective Cesarean sections (6,295/6.1%), emergency Cesarean sections (8,180/7.9%), and immediate Cesarean sections (768/0.7%). UApH median values for vaginal non-instrumental births, elective Cesarean sections, and emergency Cesarean sections were 7.240, 7.290, and 7.276 respectively. Lower median values were observed for immediate Cesarean sections (Median = 7.180), vacuum-assisted (Median = 7.170), and forceps-assisted (Median = 7.170) births. A significant negative linear relationship was identified between gestational age and UApH in vaginal non-instrumental, emergency Cesarean sections, and vacuum-assisted vaginal births (P < 0.001). Reference values were outlined for UApH based on the mode of birth and gestational age. A trend of advancing acidosis was found in infants with increasing gestational age in non-instrumental vaginal births, emergency Cesarean sections, and vacuum-assisted births. Immediate Cesarean sections and instrumental births exhibited higher levels of acidosis.