Reading Your Baby’s Blood: What Neonatal Blood Gas Tests Tell Us

Understanding why blood gas testing is so central to newborn intensive care — and what the results mean for your baby

The Most Common Test in the NICU

If your baby is in the neonatal intensive care unit (NICU), you may have noticed that healthcare staff regularly take small samples of blood and send them to a machine for analysis. These are called blood gas tests, and they are the single most frequently ordered test for babies in the NICU [[1]]. Understanding what they measure and why they matter can help you make sense of the conversations you hear at your baby’s bedside.

Put simply, a blood gas test measures how well your baby’s lungs and metabolism are working. It tells doctors and nurses whether the right amount of oxygen is getting into the blood, whether carbon dioxide (a waste gas) is being effectively removed, and whether the body’s chemistry — its acid-base balance — is in the right range. In a baby who is struggling to breathe or is unwell after a difficult birth, these numbers guide almost every decision about care.

Why Newborns Need Blood Gas Tests

When a baby is born — especially a preterm baby or one who had a difficult birth — the body faces enormous challenges in making the transition from life inside the womb to life outside it. In the womb, the placenta does the work of breathing: it delivers oxygen and removes carbon dioxide. At birth, a baby’s own lungs must take over that job almost instantly.

This transition does not always go smoothly. Premature lungs may lack surfactant (a substance that prevents them from collapsing), leading to respiratory distress syndrome. A difficult delivery may temporarily deprive the baby of oxygen. Infections can disrupt the chemistry of the blood. Without a way to measure directly what is happening inside the blood, clinicians would be working in the dark.

Blood gas testing has been used for this purpose since 1958, when researchers first showed that measuring gas levels in umbilical cord blood could reveal whether a baby had been deprived of oxygen during birth [[2]]. Armstrong and Stenson (2007) confirmed that cord blood gas, combined with other assessments, helps identify babies at risk for brain injury and allows early protective treatment [[7]]. Since then, the methods have grown more sophisticated and more gentle — but the core purpose has remained the same.

The Different Types of Blood Gas Tests

Not all blood gas tests in the NICU are the same. There are several types, each providing slightly different information and taken at different times.

Umbilical Cord Blood Gas (at Delivery)

The first blood gas test many babies have occurs right at birth, before they have even left the delivery room. The umbilical cord — the lifeline that connected your baby to the placenta — contains two arteries and one vein. After delivery, the cord is clamped and a small segment is saved for testing.

Blood in the umbilical artery carries waste gases from your baby back to the placenta. Measuring this blood gives doctors a direct “snapshot” of your baby’s metabolic state at the moment of birth [[3]]. The umbilical vein carries oxygenated blood from the placenta to the baby, so it tells a slightly different story — one more about the placenta than the baby.

This test is particularly recommended after difficult births: for example, if the baby had a low Apgar score (the quick health check done at 1 and 5 minutes after birth), if there were concerns about the baby’s heart rate during labour, or if the delivery was by emergency caesarean section [[4]].

Heel Stick (Capillary Blood Gas)

For ongoing monitoring in the NICU — not just at delivery but throughout the baby’s care — the most common method is a tiny prick to the heel. Nurses warm the baby’s heel first to encourage blood flow, then use a small automated device to make a very small puncture. A few drops of blood are collected in a thin tube and taken to a machine for analysis.

Heel sticks are much less invasive than drawing blood from an artery. Research shows that the results accurately reflect the baby’s acid-base balance and carbon dioxide levels, though they are not reliable for measuring oxygen directly [[5]]. McLain and colleagues (1988) first validated this technique in paired comparisons showing close agreement between heel-stick and arterial samples for pH and CO₂ [[8]], a finding later confirmed in preterm infants for venous and capillary measurements as well [[11]]. For oxygen monitoring, nurses use a pulse oximeter clipped to the baby’s hand or foot — continuous and non-invasive.

Studies have confirmed that using automated lancet devices — rather than older manual lancing tools — causes less pain and discomfort for the baby, takes less time, and results in fewer attempts [[5]].

Arterial Blood Gas (From a Catheter)

In the sickest or most premature infants, a fine catheter (a thin, flexible tube) is placed inside the umbilical artery or a small peripheral artery in the hand or foot. This allows nurses to draw blood directly from an artery without repeated pricks. Arterial blood gives the most complete picture, including a reliable measurement of oxygen in the blood. This catheter is also used to measure blood pressure continuously.

What the Numbers Mean

A blood gas result consists of several values. The key ones are:

pH is a measure of how acidic or alkaline the blood is, on a scale of 0 to 14. Blood should sit in a narrow healthy range — roughly 7.35 to 7.45. Too acidic (below 7.35) is called acidosis; too alkaline (above 7.45) is called alkalosis. Both can interfere with how the body functions.

CO₂ (carbon dioxide) is the main waste gas produced by the body. The lungs remove it with every breath. If CO₂ builds up (above ~45 mmHg), the blood becomes more acidic — a state called respiratory acidosis, which tells doctors the lungs are not moving enough air. This might mean a ventilator needs adjusting. If CO₂ is too low, the baby may be breathing too fast, and adjustments are made in the other direction.

Oxygen (O₂) measures how much oxygen is dissolved in the blood. In preterm babies, maintaining oxygen in the right range is critically important: too little causes harm, but too much — especially in the eyes — can contribute to a condition called retinopathy, which can affect vision [[1]]. That is why oxygen targets in the NICU are set carefully, and why continuous oxygen monitoring is so important.

Bicarbonate (HCO₃⁻) and base excess measure the kidneys’ contribution to balancing the blood’s chemistry. When these are low, it usually signals that the body has been working hard to compensate for an acid problem — often related to poor blood flow or metabolic stress.

What Blood Gas Results Tell Doctors at Delivery

If your baby needed cord blood gas testing at delivery, the results help answer one of the most important questions in newborn medicine: was there a significant deprivation of oxygen during birth?

In a healthy term baby born without complications, the umbilical artery pH is typically between 7.24 and 7.27 — already somewhat lower than normal adult blood, because the baby works hard during labour and delivery [[3]]. Values in this range are reassuring.

A pH below 7.00 combined with a base deficit greater than 12 mEq/L and other signs of distress — low Apgar scores, need for resuscitation — is a significant warning sign. Goodwin and colleagues (1992) found that severe umbilical acidaemia at this level is associated with serious complications in term newborns [[9]], and Perlman and Risser (1996) showed that babies at risk for seizures can be identified rapidly at delivery using these combined criteria [[10]]. About 80% of affected non-vigorous infants develop seizures in the first days of life, and some are at risk for hypoxic-ischaemic encephalopathy (HIE) — brain injury from oxygen deprivation [[3]].

A large review published in the BMJ in 2010, covering more than 400,000 births across 51 studies, confirmed that a low cord blood pH is consistently and meaningfully associated with poor outcomes when combined with other abnormal findings [[6]]. This evidence is one reason why cord blood gas testing is now standard practice in high-risk deliveries.

Importantly, a low pH on its own in a vigorous, pink, crying baby is much less concerning. Most babies with isolated low cord pH — without any other signs of distress — do fine without any special treatment [[3]].

Making the Test Less Painful

One of the most visible challenges in neonatal intensive care is balancing the medical necessity of blood tests against the discomfort they cause. Babies feel pain, and repeated painful procedures in early life can have a sensitising effect — later tests may cause more distress than earlier ones [[5]].

Neonatal teams work hard to minimise this burden. Evidence from clinical research supports several effective strategies for reducing heel stick pain: giving a small amount of oral sugar solution (sucrose) before the procedure, offering non-nutritive sucking (a dummy or pacifier), swaddling the baby, offering breast milk, and gentle touch or skin-to-skin care [[5]]. Automated lancet devices also meaningfully reduce pain, procedure time, and the number of attempts compared with manual ones.

If your baby is having frequent blood gas tests, it is entirely appropriate to ask the team how they manage pain during sampling and what you can do to help.

Looking After the Blood Volume

A point that surprises many parents is how carefully the NICU team tracks how much blood is taken from a baby. A 600-gram premature baby has a total blood volume of only about 50 to 60 millilitres — roughly the volume of a small shot glass [[1]]. Even one millilitre represents about 2% of that total. Modern NICU analysers require very small samples (some need as little as 65 microlitres), and teams carefully limit draws to only what is clinically necessary.

What Comes Next in Blood Gas Research

Blood gas testing in newborns is a mature field, but research continues to refine it. Scientists are investigating whether measuring lactate — a chemical produced when cells are deprived of oxygen — can supplement or replace cord pH in identifying babies at risk for brain injury [[3]]. Other work focuses on how fetal haemoglobin — the special form of haemoglobin in newborns — affects oxygen readings, and on how to minimise the number of blood tests needed without compromising care.

A Final Word for Families

Blood gas testing can seem alarming — any test that involves drawing blood from your tiny baby raises natural concerns. But these tests are central to safe, evidence-based neonatal care, and the teams performing them are trained to do so as gently as possible.

If you ever want to understand a result your baby’s care team has mentioned, or why a ventilator setting has been adjusted after a blood gas, please ask. The NICU team is there not only to care for your baby, but to help you understand and be part of that care.

References

  1. Mulligan M. Blood gas interpretation in the neonate — what do you need to know now? Acutecaretesting.org. January 2013. Available at: <https://acutecaretesting.org/en/articles/blood-gas-interpretation-in-the-neonate> ↩
  2. James LS, Weisbrot IM, Prince CE, Holaday DA, Apgar V. The acid-base status of human infants in relation to birth asphyxia and the onset of respiration. J Pediatr. 1958;52(4):379–394. PMID: 13539725. ↩
  3. Saneh H, Mendez MD, Srinivasan VN. Cord Blood Gas. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 Jan–. PMID: 31424874. Available at: <https://www.ncbi.nlm.nih.gov/books/NBK545290/> ↩
  4. ACOG Committee on Obstetric Practice. ACOG Committee Opinion No. 348, November 2006: Umbilical cord blood gas and acid-base analysis. Obstet Gynecol. 2006;108(5):1319–1322. doi:10.1097/00006250-200611000-00058 ↩
  5. Evans DL, Volsko TA, Capellari E, Strickland SL. AARC Clinical Practice Guidelines: Capillary Blood Gas Sampling for Neonatal and Pediatric Patients. Respir Care. 2022;67(9):1190–1204. doi:10.4187/respcare.10151 ↩
  6. Malin GL, Morris RK, Khan KS. Strength of association between umbilical cord pH and perinatal and long term outcomes: systematic review and meta-analysis. BMJ. 2010;340:c1471. doi:10.1136/bmj.c1471 ↩
  7. Armstrong L, Stenson BJ. Use of umbilical cord blood gas analysis in the assessment of the newborn. Arch Dis Child Fetal Neonatal Ed. 2007;92(6):F430–F434. doi:10.1136/adc.2006.099846 ↩
  8. McLain BI, Evans J, Dear PR. Comparison of capillary and arterial blood gas measurements in neonates. Arch Dis Child. 1988;63(7 Spec No):743–747. doi:10.1136/adc.63.7_Spec_No.743 ↩
  9. Goodwin TM, Belai I, Hernandez P, Durand M, Paul RH. Asphyxial complications in the term newborn with severe umbilical acidemia. Am J Obstet Gynecol. 1992;167(6):1506–1512. doi:10.1016/0002-9378(92)91736-z91736-z) ↩
  10. Perlman JM, Risser R. Can asphyxiated infants at risk for neonatal seizures be rapidly identified by current high-risk markers? Pediatrics. 1996;97(4):456–462. PMID: 8632928. ↩
  11. Tan RNGB, Pauws SC, van Loon E, Smits VEHJ, Lopriore E, Te Pas AB. Correlation and interchangeability of venous and capillary blood gases in non-critically ill neonates. Front Pediatr. 2018;6:89. doi:10.3389/fped.2018.00089 ↩