Umbilical Cord Blood Gases and Hypoxic-Ischemic Encephalopathy | Birth Injury Diagnosis

Michigan birth injury attorneys serving clients across the U.S.

Michigan birth injury attorneys serving clients across the U.S.

Umbilical cord blood gas tests can be very important in diagnosing birth injuries such as hypoxic-ischemic encephalopathy (HIE), a form of neonatal brain damage that can occur when the baby’s brain does not receive enough oxygenated blood.

At ABC Law Centers (Reiter & Walsh, P.C.), we aim not only to provide unparalleled legal services to our clients, but to make complex information on birth injury easier to access. Throughout this page, we’ll discuss fetal circulation, oxygen deprivation/birth asphyxia, neonatal brain damage, and the role of umbilical cord blood gas tests in birth injury diagnoses and litigation. Should you have any legal questions or case inquiries as you read through this page, please contact our team for a free consultation.

Fetal circulation: understanding how babies breathe in the womb

fetal circulation; placenta; umbilical cord; fetal oxygenation

In the womb, a fetus does not breathe in the same way humans do outside the womb (although “practice breathing” occurs from time to time [1]). Rather, oxygen molecules must traverse a pathway involving the mother’s lungs, heart, vasculature, uterus, and placenta, before finally making its way through the umbilical cord and into the fetus (2). Carbon dioxide and other fetal waste products travel back through the umbilical cord and placenta, into the mother’s circulatory system (3). At any point along this pathway, gas exchange between mother and baby can be dangerously interrupted. In many cases, birth asphyxia and subsequent issues (such as hypoxic-ischemic encephalopathy, or HIE) stem from an interruption in one of the final three steps: the uterus, placenta, or umbilical cord. Umbilical cord blood gas tests can provide valuable information on how such interruptions affect individual babies.

placenta; fetal oxygenation; pregnancy; umbilical cord

Umbilical cord blood: venous vs. arterial

After birth, umbilical cord blood tests can provide valuable information on the baby’s status. However, it is important to note that results may vary depending on what part of the umbilical cord blood is drawn from.

The umbilical cord contains one large vein, which carries blood from the placenta to the fetus, and two small arteries, which carry blood from the fetus back to the placenta (4). Blood in the umbilical vein primarily reflects placental status. This does of course influence fetal status (because the fetus gets oxygen and nutrients from the placenta), but blood in the umbilical arteries reflects fetal status more directly. Therefore, it may be more useful to sample arterial blood if seeking information about whether the baby has experienced a hypoxic-ischemic injury. If umbilical artery blood has a low pH (is acidic), that means that the baby experienced fetal acidosis/anaerobic metabolism (5). Anaerobic metabolism occurs when oxygen is not readily available, and is therefore an indicator that an hypoxic-ischemic or anoxic event occurred. These oxygen-depriving events can cause a condition called hypoxic-ischemic encephalopathy (HIE), which is a form of neonatal brain damage that can lead to cerebral palsy, seizure disorders, and other permanent conditions.

Typical values for umbilical artery samples

Different studies have reported slightly different averages and ranges for umbilical artery blood samples. The following data came from a 1993 study by Anthony R. Gregg and Carl P. Weine (6):

Normal values in an umbilical arterial sample in a term newborn:

  • Mean pH: 7.28 (standard deviation 0.07)
  • Mean PCO2: 49.9 (standard deviation 14.2)
  • Mean PO2: 23.7 (standard deviation 10.0)
  • Mean HCO3: 23.1 (standard deviation 2.8)
  • Mean base deficit: -3.6 (standard deviation 2.8)

Normal umbilical arterial values in a preterm newborn:

  • Mean pH: 7.29 (standard deviation 0.07)
  • Mean PCO2: 49.2 (standard deviation 9.0)
  • Mean PO2: 23.6 (standard deviation 8.9)
  • Mean HCO3: 23.0 (standard deviation 3.5)
  • Mean base deficit: -3.3 (standard deviation 2.4)

*The “P” in PCO2 and PO2 stands for “partial pressure,” which is how these umbilical cord blood gases are measured.

Interpreting umbilical cord blood gases and detecting birth asphyxia

The most important measurements in a blood gas test for evaluating a baby’s current condition and prognosis are the pH and the base deficit.

Some experts define fetal acidemia  as a pH of less than 7.1. However, there may be negative consequences even with a higher pH (this is discussed more below; see “Severity of acidosis and brain damage”).

While acidosis can be recognized using only pH, the base deficit (calculated from the pH and the partial pressure of carbon dioxide, or PCO2) can help to determine whether that acidosis was respiratory or metabolic. With either type of acidosis, outcomes can vary; some children will have no lasting neurological issues, while others may develop serious complications. However, metabolic acidosis is more likely to be associated with negative outcomes than is respiratory acidosis. If there is a base deficit of 12 mmol/L or above, this suggests metabolic acidosis. Infants with metabolic acidosis are at higher risk for the following complications:

  • Multiorgan failure
  • Death
  • Moderate-to-severe neonatal encephalopathy (brain damage)
  • Long-term neurological conditions

Lactate levels may also serve as a marker of fetal acidosis (5).

Causes of acidosis

Fetal hypoxemia occurs in three ways, which we will explain in detail below.

1. Reduced delivery of oxygen to the placenta

The following complications can cause the placenta to receive insufficient oxygen, and lead to fetal acidosis:

2. Reduced transfer of oxygen across the placenta from the maternal to the fetal side

When reduced levels of oxygen transfer from the mother to the fetus through the placenta, fetal acidosis can occur. Conditions that can cause this include the following:

3. Reduced transport of oxygen by the fetal circulation

Decreased fetal oxygen transport can occur when the following conditions are present:

  • Umbilical cord complications (such as a nuchal cord, pictured below) leading to cord compression
  • Fetal anemia from rhesus disease, parvovirus infection, α-thalassaemia or feto-maternal haemorrhage
  • Fetal tumors
  • Serious cardiac problems (7, 8)
Nuchal Cord and Hypoxic Ischemic Encephalopathy (HIE)

Severity of acidosis and brain damage

In 2010, authors of an important review paper (9) concluded that low arterial pH in umbilical cord blood was strongly associated with long-term adverse outcomes. These outcomes included hypoxic-ischemic encephalopathy (HIE), periventricular leukomalacia (PVL), intraventricular hemorrhages, cerebral palsy (CP), and death.

What was unclear, at that point, was what pH constituted clinically significant acidemia. A 2012 study (10) of singleton, term neonates found the ideal cord arterial pH to be 7.26 – 7.30 for all outcomes. The risk for adverse neurological outcomes starts to rise below a pH of 7.10, with the risk being highest below a pH of 7.0. Another key finding of this study is that most neonates with adverse outcomes, even that of seizures in the first 24 hours, are not born acidemic. In addition, the authors concluded that “it appears that the lowest risk of any adverse outcome occurs at 7.26 – 7.30, rather than ‘the higher the better’, for there may be a higher risk at higher pH levels.”

The American College of Obstetricians and Gynecologists (ACOG) suggests that the arterial cord blood’s pH should be less than 7.0 if it is going to be used as a factor establishing a link between birth asphyxia and neurological injury (11). This conclusion, however, is debatable. Multiple studies confirm that birth asphyxia can cause brain damage even when umbilical cord arterial pH is greater than 7.0. Yeh et al (whose study excluded infants with major congenital abnormalities) found that infants with a pH between 7.0 and 7.11 accounted for 10 – 15% of adverse neurological outcomes in their study. Even when considering cases involving seizures in the first 24 hours of life, at least half had a pH above 7.10, and 39 percent had a pH over 7.20.

Moreover, Yeh et al. noted the following (10):

  • Many babies with birth asphyxia (who often have low Apgar scores) often have a normal pH. (A 2012 study by Joseph J. Volpe also noted this [12])
  • In some cases, neonates with low Apgar scores who are acidemic may do better in the long term than those who are not.
  • Catastrophic intrapartum events can occur without acidemia.

The aforementioned events can occur because newborns who experienced hypoxia may have been unable to develop acidemia as a response. Yeh et al. suggest that this can occur if cerebral circulation is disrupted, for reasons such as shoulder dystocia or umbilical cord occlusion.

Artery cord blood samples may significantly underestimate fetal or newborn acidosis

Severely asphyxiated newborns may have poor perfusion around the time of birth; this can include umbilical circulation. If blood flow from the fetus through the umbilical cord has been disrupted, the umbilical cord blood will not accurately reflect fetal status. As the baby is resuscitated, circulation improves and tissue lactic acid is cleared into the central circulation. For this reason, postnatal blood gas tests may reveal worse acidosis than was found in the umbilical cord blood test. These postnatal tests can be important predictors of neurological outcome (13).

Umbilical cord blood gases and birth injury litigation

Umbilical cord blood gases are frequently used in birth injury litigation. A hospital may attempt to use normal umbilical cord gas results to defend their case on causation. There are many reasons why a fetus who suffered asphyxia or hypoxia can have a normal cord gas. The fetus may have had very poor circulation and perfusion right before birth, the fetus may have suffered a severe and sudden head injury during delivery that caused ischemia (lack of blood flow) in the brain, or the results may be invalid due to error in drawing, storing or analyzing the blood. A 2002 study suggested that up to 19 percent of umbilical cord blood samples may be invalid due to technical errors (14).

Due to the importance umbilical cord gas results play in litigation, it is imperative to have an attorney with experience in this area. A skilled birth injury attorney will know what to look for in medical records to determine if a sample is valid and an accurate reflection of the baby’s condition at the time of birth. They will also work with the best experts to find the cause and timing of the birth injury.

Legal help for birth injury and hypoxic-ischemic encephalopathy (HIE)

At ABC Law Centers (Reiter & Walsh, P.C.), our award-winning attorneys have over 130 years of joint experience with birth injury cases. With the aid of our registered nursing team, we will research your case, interpret your loved one’s medical records, and determine the cause of the injuries. For decades, we have been helping families in Michigan and throughout the nation, and we have dozens of multi-million dollar verdicts and settlements that attest to our success in the field of birth trauma litigation. We will fight to obtain the compensation your child deserves for lifelong care, treatment, and therapy. Clients pay nothing unless we obtain a verdict or settle in their favor!

  • Free Case Review
  • Available 24/7
  • No Fee Unless We Win

Featured Videos

Testimonial from Keziah’s Family

Posterior Position

Hypoxic-Ischemic Encephalopathy (HIE)

quotation mark

Featured Testimonial

What Our
Clients Say…

After the traumatic birth of my son, I was left confused, afraid, and seeking answers. We needed someone we could trust and depend on. ABC Law Centers was just that.

- Michael

Helpful resources

When you see your child suffer because of someone’s mistake you want justice. As a mother you want someone in your corner to fight as hard as you would and I luckily found that…Is my son cured because of Jesse and his staff? No, but my son will now have better future because of them!” –Wendy

Read more of our client testimonials here.

  1. How Do Babies Breathe in the Womb? (n.d.). Retrieved April 10, 2019, from
  2. Reiter, J. (2017, September 25). How Do Babies Breathe in the Womb? Retrieved April 10, 2019, from 
  3. Children’s Hospital. (2014, August 24). Blood Circulation in the Fetus and Newborn. Retrieved April 10, 2019, from
  4. Fetal Life-Support System: Placenta and Umbilical Cord. (2017, April 18). Retrieved April 10, 2019, from
  5. (n.d.). Retrieved April 10, 2019, from
  6. Gregg, A. R., & Weiner, C. P. (1993). “Normal” umbilical arterial and venous acid-base and blood gas values. Clinical obstetrics and gynecology, 36(1), 24-32.
  7. Bobrow, C. S., & Soothill, P. W. (1999). Causes and consequences of fetal acidosis. Archives of Disease in Childhood-Fetal and Neonatal Edition, 80(3), F246-F249.
  8. (n.d.). Retrieved April 10, 2019, from
  9. Malin, G. L., Morris, R. K., & Khan, K. S. (2010). Strength of association between umbilical cord pH and perinatal and long term outcomes: systematic review and meta-analysis. BMJ, 340, c1471.
  10. Yeh, P., Emary, K., & Impey, L. (2012). The relationship between umbilical cord arterial pH and serious adverse neonatal outcome: analysis of 51 519 consecutive validated samples. BJOG: An International Journal of Obstetrics & Gynaecology, 119(7), 824-831.
  11. Schendel, D. (2014). Executive summary: neonatal encephalopathy and neurologic outcome, Report of the American College of Obstetricians and Gynecologists’ task force on neonatal encephalopathy. Obstetrics and Gynecology, 123(4), 896-901.
  12. Volpe, J. J. (2012). Neonatal encephalopathy: an inadequate term for hypoxic–ischemic encephalopathy. Annals of Neurology, 72(2), 156-166.
  13. Casey, B. M., Goldaber, K. G., McIntire, D. D., & Leveno, K. J. (2001). Outcomes among term infants when two-hour postnatal pH is compared with pH at delivery. American Journal of Obstetrics and Gynecology, 184(3), 447-450.
  14. Tong, S., Egan, V., Griffin, J., & Wallace, E. M. (2002). Cord blood sampling at delivery: do we need to always collect from both vessels?. BJOG: An International Journal of Obstetrics & Gynaecology, 109(10), 1175-1177.