What is the rate of umbilical cord artery pH less than 7 at full dilation, i.e., during the secondary stage of labor?

From the Guidelines

The rate of cord artery pH less than 7 at full cervical dilation (secondary stage of labor) is approximately 0.5-1% of all deliveries, as indicated by studies on intrapartum fetal monitoring 1. This represents severe fetal acidemia, which is a concerning finding that indicates significant fetal hypoxia during labor. The normal umbilical cord arterial pH range is typically 7.18-7.38, with values below 7.0 considered pathological acidemia. Some key points to consider in this context include:

• Severe acidemia can occur due to various factors during labor, including prolonged second stage, umbilical cord compression, placental insufficiency, or uterine hyperstimulation.
• Monitoring for fetal distress during labor through continuous electronic fetal monitoring and, when indicated, fetal scalp blood sampling can help identify fetuses at risk before pH drops to these critical levels.
• Prompt intervention when signs of fetal distress are detected, including expedited delivery or intrauterine resuscitation measures, can help prevent progression to severe acidemia, as outlined in interventions for abnormal electronic fetal monitoring 1. The presence of cord arterial pH below 7.0 is associated with increased risk of neonatal complications including hypoxic-ischemic encephalopathy, seizures, and multiorgan dysfunction. Key interventions to consider in the case of abnormal fetal heart rate tracings include:
• Changing maternal position
• Assessing maternal vital signs
• Discontinuing oxytocin infusion
• Initiating oxygen therapy
• Performing a vaginal examination
• Giving intravenous fluids
• Assessing fetal pH
• Considering the need for expedited delivery, as outlined in Table 5 of the intrapartum fetal monitoring study 1.

From the Research

Rate of Cord Artery pH Less Than 7 at Full Dilation

• The rate of cord artery pH less than 7 at full dilation, ie secondary stage, is reported in a study published in 2023 2.
• According to this study, the rate of umbilical cord metabolic acidemia at birth (MA; pH < 7.0) was 3.32% (115/3459) in patients experiencing the 2nd stage of labor.
• The study also found that the cumulative incidence function (CIF) at 180 minutes in the 2nd stage was 2.67% for nonpathological and 10.63% for pathological CTG_RED.
• Another study published in 2023 3 reported that among 248 infants with cord arterial pH <7,89.5% were intact survivors and 10.5% had poor outcomes, including neonatal/infant deaths or cerebral palsy and/or developmental delay.
• The risk of adverse outcome increased from 4.3% when the arterial pH was between 6.9 and <7, to 30% when the pH was <6.9 3.
• A study published in 2017 4 found no significant difference in umbilical cord artery pH between women who received supplemental oxygen during the second stage of labor and those who did not.

Predictors of Adverse Outcomes

• Nulliparity, pathological CTG, and meconium-stained amniotic fluid were significant predictors of MA in a multivariable model 2.
• Cord arterial pH of <6.9 and an Apgar score at 5 minutes <7 were independent prognostic factors for neonatal/infant death or adverse long-term neurological outcomes 3.