Umbilical Cord Blood Gases: What the Numbers in a Delivery Record Show

Modern obstetrics gives delivery teams many tools for watching over a baby during labor, from continuous electronic fetal monitoring to bedside ultrasound and rapid laboratory testing. That said, most of what happens in a delivery room is recorded through observation and interpretation, and only a handful of measurements record the baby's condition in hard numbers at the moment of birth.

Umbilical cord blood gases are among those few objective measurements. When a sample is drawn from a clamped segment of the cord and run through a blood gas analyzer, the results show how much acid had built up in the baby's blood, whether that acid was respiratory or metabolic in origin, and how severe the stress before delivery may have been.

For families trying to understand a difficult birth, those numbers can read like a foreign language. We understand how disorienting it is to sort through a delivery record full of abbreviations while also caring for a newborn who needed resuscitation, cooling therapy, or a stay in the NICU.

This guide explains what the values mean, how physicians and retained experts read them alongside the fetal heart rate tracing, and why obtaining the laboratory records early matters to what an Illinois claim must prove. It is part of our In the Delivery Room library, which covers the decisions and measurements made between admission and the first minutes of a newborn's life.

During pregnancy and labor, the placenta handles the baby's gas exchange — delivering oxygen and carrying away carbon dioxide and acid. When oxygen delivery falls, the baby's cells shift toward anaerobic metabolism, and the byproducts of that shift accumulate in the blood, where a cord sample can detect them.

Ideally, the delivery team double-clamps a segment of the cord immediately after birth and draws blood from both the umbilical artery and the umbilical vein. The American College of Obstetricians and Gynecologists has described paired arterial and venous sampling as the preferred approach, and a properly clamped segment stays stable long enough for the team to focus on the newborn first.

Keep in mind that the two vessels tell different stories. The umbilical arteries carry blood returning from the baby, so arterial values reflect the baby's own condition, while the umbilical vein carries blood coming back from the placenta and reflects how well the placenta was doing its job.

A typical cord gas report includes several values, including but not limited to:

• pH. This is the overall measure of acidity in the sample, and lower numbers mean more acid. Because the pH scale is logarithmic, a drop that looks small on paper can represent a substantial change in the baby's blood chemistry.
• pCO2. The partial pressure of carbon dioxide shows how much carbon dioxide had built up, and it rises quickly when gas exchange is interrupted. An elevated pCO2 alongside an otherwise reassuring profile often points to a brief, recent event.
• pO2. The partial pressure of oxygen reflects oxygen levels in the sample at the time of collection. Experts generally treat it as less informative than the other values because it changes so quickly once the cord is cut.
• Base deficit (or base excess). This calculated value estimates how much of the blood's buffering capacity has been used up by metabolic acid. A rising base deficit is one of the clearest markers that oxygen deprivation lasted long enough to change the baby's metabolism.
• Lactate. Lactate is a direct byproduct of anaerobic metabolism, so it rises when tissues are working without enough oxygen. Many hospital laboratories now report it alongside the standard gas panel.

Together, these values form a snapshot of the baby's acid-base status at the moment of birth. That snapshot cannot replay the hours of labor, but it gives everyone reviewing the record a fixed point against which the rest of the chart can be measured.

ACOG guidance has also identified circumstances in which cord gases should be obtained, such as a cesarean delivery for fetal compromise, a low five-minute Apgar score, an abnormal fetal heart rate tracing, severe growth restriction, intrapartum fever, and multiple gestation. Whether a sample was drawn when the circumstances called for one can itself become part of the review.

Reading a cord gas starts with confirming that the sample is what it claims to be. If the arterial and venous values are nearly identical, an expert may suspect that both samples came from the vein — a recognized collection error — and delays in analysis or air bubbles in the syringe can also distort the results.

Once the sample is validated, the next question is the type of acidosis. Experts generally sort cord gases into three broad patterns:

• Respiratory acidosis. The pCO2 is elevated while the base deficit stays within a normal range. This pattern usually reflects a short interruption in gas exchange, such as cord compression in the final minutes before delivery, and it often corrects quickly once the baby begins breathing.
• Metabolic acidosis. The base deficit and lactate are elevated, showing that tissues had been forced into anaerobic metabolism. This pattern suggests a more sustained or severe loss of oxygen, and it is the pattern most closely examined in hypoxic-ischemic injury cases.
• Mixed acidosis. Both carbon dioxide and metabolic acid are elevated. Mixed patterns are common, and experts look at the relative weight of each component to understand what happened and when.

The thresholds matter as well. In its report on neonatal encephalopathy, a joint task force of ACOG and the American Academy of Pediatrics identified an umbilical arterial pH below 7.0, a base deficit of 12 mmol/L or greater, or both, as findings consistent with an acute event around the time of labor and delivery.

Those thresholds are reference points rather than bright lines. Many babies with significant acidemia recover without lasting injury, and some babies with serious brain injury have cord values that fall short of those cutoffs, which is why an expert never reads the numbers in isolation.

Lactate adds another layer. There is no single universally adopted lactate cutoff, so experts typically compare the result to the reporting laboratory's reference range and to the base deficit, looking for consistency between the two.

Finally, the gap between arterial and venous values can say something about mechanism and timing. For instance, a wide difference between the two can point toward an acute obstruction of cord blood flow, while values that are both abnormal and close together may suggest a problem with placental function itself — interpretations that a qualified expert must weigh against the full record.

Cord gases show where the baby ended up, and the fetal heart rate tracing shows how the baby got there. An expert's task is to line the two up and ask whether the tracing's progression is consistent with the degree of acidosis found at birth.

Electronic fetal monitoring is interpreted using the three-tier category system developed through a National Institute of Child Health and Human Development workshop and adopted by ACOG. Category I tracings are considered normal, Category III tracings are considered abnormal and call for prompt evaluation and intervention, and Category II covers the broad indeterminate middle ground that requires ongoing surveillance and reassessment.

When reviewing the tracing against the cord gas, experts look closely at several features, such as:

• Baseline variability. Moderate variability is generally reassuring, while minimal or absent variability, particularly when combined with recurrent decelerations, can indicate that the baby's ability to compensate is declining.
• Recurrent late decelerations. Decelerations that follow contractions can signal reduced oxygen reserve. Experts note when they began and how the clinical team responded.
• Prolonged decelerations and bradycardia. A sustained drop in the heart rate shortly before delivery may help explain a severe metabolic acidosis at birth. The interval from the onset of a bradycardia to delivery often becomes a central question.
• Loss of accelerations and changes in baseline. A rising baseline rate or disappearing accelerations can be part of a gradual deterioration. Seen together, these changes help an expert estimate when the baby's reserves started to run out.

This is why the timeline matters so much. If a tracing showed concerning features for an extended period and the cord gas confirms metabolic acidosis, the expert review will focus on whether the clinical team recognized those features and acted — for example, through intrauterine resuscitation, escalation to a physician, or a timely decision to deliver rather than a delayed emergency C-section.

Our guide to fetal monitoring failures explains how those missed-recognition questions are analyzed. In many cases, the cord gas is what confirms whether the warning signs on the strip reflected real and worsening oxygen deprivation.

The cord gas is also read alongside the newborn's early course. Apgar scores, the resuscitation record, the first neonatal blood gas, seizure activity, the decision to begin therapeutic hypothermia, placental pathology, and early brain imaging all help determine whether the findings fit the pattern of HIE and oxygen deprivation.

Of course, the comparison can cut in more than one direction. If the tracing was reassuring until moments before birth and the cord gas shows only respiratory acidosis, the record may point elsewhere — toward an earlier antepartum event, infection, or a postnatal complication — and an honest review follows the evidence wherever it leads.

Cord gas results do not always live in one place. They may appear in the laboratory section of the chart, the labor and delivery nursing notes, the newborn's own record, or only on the printout generated by the blood gas analyzer.

As a result, a general request for the mother's chart may not produce everything. A thorough request for a birth injury review typically includes:

• Both charts. The mother's labor and delivery record and the newborn's chart are separate records. Each may hold different pieces of the cord gas story.
• Laboratory source data. This includes the laboratory report with specimen labels, collection and result times, and any notes about sample type or quality. Analyzer printouts or instrument logs can help confirm which vessel each sample came from.
• The complete fetal monitoring strip. The full electronic tracing, with annotations, should be requested in its complete form. Selected excerpts can leave out the very stretch an expert needs to see.
• Electronic audit trails. Audit trails show when entries were created, viewed, or changed. They can matter when the timing of a result or a late entry is in question.
• Placental pathology. If the placenta was sent to pathology, the report and, where available, the slides can help explain whether a problem was acute or longstanding. Placental findings are often read side by side with the cord gas.

Note that evidence in a birth injury matter does not wait indefinitely. Hospitals set their own retention schedules, analyzer memory may be overwritten as new samples are run, and placental tissue may not be kept, so the time pressure here sits on the evidence rather than on any family.

Under the federal HIPAA right of access, patients and their personal representatives can generally obtain copies of their medical records, and parents typically act as personal representatives for a minor child. Illinois also has its own records statute, 735 ILCS 5/8-2001, which governs requests to health care facilities and practitioners, and how these rules apply to a particular request is worth confirming with counsel.

An Illinois medical negligence claim arising from a birth generally must establish the standard of care, a deviation from that standard, causation, and damages. Cord gases rarely answer the standard-of-care question by themselves, but they often sit at the center of the causation and timing analysis.

Put simply, a case must show that the baby was harmed, that the harm is connected to care during labor and delivery, and that timely action would more likely than not have changed the outcome. A metabolic acidosis that lines up with a deteriorating tracing can support that connection, while a cord gas that points to an earlier or unrelated process can weaken it.

Illinois also imposes a threshold requirement before most medical negligence cases can proceed. Under 735 ILCS 5/2-622, the plaintiff's attorney generally must file an affidavit stating that a qualified health professional has reviewed the records and concluded in a written report that there is a reasonable and meritorious cause for filing the action.

That reviewing health professional will look closely at the cord gas, the tracing, and the newborn's course. Accordingly, an incomplete set of laboratory records can make the early review harder, and it is one more reason the source data matters.

Timing rules for filing are a separate and fact-specific question. Illinois sets limitations and repose periods for medical claims in 735 ILCS 5/13-212, and the answer can differ for the child's own claim, a parent's claim for their own losses, and the outer repose period, so the deadline depends on the facts and should be confirmed with counsel — our guide to bringing a claim in Illinois walks through those distinctions.

Where the evidence supports a connection to an injury such as cerebral palsy or other neonatal injuries, damages may include the cost of lifetime care, therapies, equipment, and lost earning capacity. Those needs are typically documented through treating specialists and life care planners.

We understand that no review of numbers can undo what a family has been through, and that many parents simply want to know whether the outcome could have been prevented. Our team works with physicians and other medical professionals to read cord gases, tracings, and newborn records together, and we welcome the opportunity to help a family understand what those records show.

A consultation is free and confidential, and there is no fee unless we recover compensation. To explore the other topics in our library, visit the Chicago Birth Injury Lawyers hub.

This article is for informational purposes and does not constitute legal or medical advice. Consult a qualified attorney in Illinois and a licensed clinician about your specific situation.