Umbilical cord blood gas testing uses arterial and venous cord samples to assess fetal oxygenation and acid-base balance at birth. Measured values (pH, PCO2, PO2, bicarbonate, base-excess, sometimes lactate) help distinguish respiratory versus metabolic causes of acidosis. Indications include nonreassuring fetal heart tracings, low Apgar scores, significant resuscitation, meconium with depression, or major maternal events. Proper double-clamping, rapid labeling, and prompt analysis are important. Results inform but do not by themselves determine cause or timing of fetal compromise.

Why cord blood gases matter

The placenta delivers oxygen and removes carbon dioxide for the fetus. After delivery, testing blood from the umbilical cord gives a direct biochemical snapshot of the fetal condition at birth. Clinicians use cord blood gas results to document oxygenation and acid-base status and to help decide immediate newborn care.

What is measured

Laboratories typically report pH, partial pressures of carbon dioxide (PCO2) and oxygen (PO2), bicarbonate (HCO3-), and a base-excess/base-deficit value. Some centers also measure lactate as an additional marker of metabolic stress. The umbilical arterial sample reflects fetal metabolic status more directly; the venous sample primarily reflects placental transfer from the mother.

How to collect and handle samples

Teams commonly double-clamp the cord and collect paired arterial and venous samples, labeling each clearly. Proper technique helps avoid maternal blood contamination. Cord gases should be analyzed rapidly or stored on ice and sent to the lab quickly because delays change values.

How to read the results (practical rules)

  • Low pH with elevated PCO2 generally indicates respiratory acidosis: the fetus or newborn was hypoventilating or had impaired gas exchange.
  • Low pH with low bicarbonate and a large negative base-excess (sometimes reported as base deficit) indicates metabolic acidosis from tissue hypoxia and anaerobic metabolism.
  • PO2 in cord blood is normally lower than adult arterial PO2 and is less useful than pH and base-excess for judging perinatal hypoxia.
Cord arterial pH and base-excess are the measurements clinicians rely on most to assess the severity of intrapartum acidemia and to inform interventions. They do not, however, identify the precise cause of an abnormality (for example, placental abruption, prolonged cord compression, or maternal illness) without correlation to the clinical context.

Common clinical indications for cord blood gas testing

  • Nonreassuring fetal heart rate patterns during labor.
  • A newborn with low Apgar scores or one who required significant resuscitation at birth.
  • Suspected intrapartum hypoxia or asphyxia (for documentation and to help decide therapies such as therapeutic hypothermia in appropriate cases). 1
  • Meconium-stained fluid with neonatal depression, or significant maternal events (e.g., placental abruption, massive hemorrhage).
Routine cord gas testing is not needed for every normal delivery, but many hospitals collect samples when a baby requires resuscitation or when delivery circumstances are high risk.

Limitations

Cord blood gas results provide objective information about acid-base status at birth but cannot alone determine the timing or exact cause of injury. Clinical context, fetal monitoring, and newborn examination remain essential.

If results are abnormal, neonatal teams will manage respiratory support, fluid and electrolyte abnormalities, and temperature control, and may request additional tests (blood glucose, electrolytes, lactate, or imaging) as indicated.

  1. Confirm recommended maximum time between cord clamping and sample analysis or the accepted storage protocol for cord gas samples in hospital labs.
  2. Verify commonly used numeric thresholds for defining severe acidemia (e.g., arterial pH cutoff) and base-deficit thresholds used to select newborns for therapeutic hypothermia.
  3. Confirm current guideline language (ACOG/AAP or equivalent) about routine versus selective cord gas sampling and indications for collection.

FAQs about Umbilical Cord Blood Gas

What is the difference between umbilical arterial and venous blood gases?
Umbilical arterial blood reflects the fetal circulation and is most useful for assessing fetal metabolic status and acidemia. Umbilical venous blood reflects placental transfer from the mother and is less specific for fetal distress, but paired samples help confirm correct sampling.
When should cord blood gases be collected?
Collect cord gases when the clinical picture suggests possible fetal compromise - examples include nonreassuring fetal heart tracings, low Apgar scores, need for extensive neonatal resuscitation, meconium with a depressed infant, or major maternal events like placental abruption. Many centers also collect when therapeutic decisions (e.g., cooling) may depend on the results.
What does a low cord pH mean?
A low pH indicates acidemia. If paired with high PCO2 it suggests respiratory acidosis; if accompanied by low bicarbonate and a large base deficit it suggests metabolic acidosis from tissue hypoxia. The pattern helps clinicians decide immediate newborn management.
Can cord blood gases identify the cause of a baby’s difficulties at birth?
No. Cord gases document the acid-base status at birth but do not identify the specific cause or timing of an event. Clinical history, fetal monitoring, and newborn exam are necessary to interpret the results.
How soon should cord blood gas samples be analyzed?
Samples should be analyzed promptly or stored on ice and delivered to the lab quickly; delays can alter results. Exact storage and timing procedures vary by hospital and lab protocols.