Blood Gas: Fetal


Umbilical artery

Umbilical vein




PCO2 (mmHg)



pO2 (mmHg)



Bicarbonate (mEq/L)



Base excess (mEq/L)



  • During oxidative metabolism, carbonic acid is produced, which is usually cleared by the placenta as carbon dioxide [2]. If placental blood flow is not adequate, then CO2 elimination can be affected leading to respiratory acidosis. Lactic and beta-hydroxybutyric acids are produced as a result of anaerobic metabolism [3], which requires hours of metabolic clearance and contributes to metabolic and mixed acidosis.

  1. 2.

    Fetal acid–base balance can be accessed via a number of ways:

    1. (a)

      Antepartum: by percutaneous umbilical cord blood sampling


    2. (b)

      Intrapartum: by fetal scalp blood sampling (after membranes have ruptured)


    3. (c)

      Postpartum: by umbilical cord blood sampling



  2. 3.

    Usually, blood samples from both umbilical artery and vein are collected, which represent the fetal and maternal condition, respectively. In addition to maternal condition, umbilical vein blood samples also represent the utero-placental gas exchange.

    In order for blood samples to be accurate, the umbilical cord should be double clamped at least 10–20 cm apart immediately after delivery, and the blood samples should be drawn via heparinized syringe within 15 min of delivery [3]. For accuracy, the samples should be analyzed within 30–60 min. Air bubbles should also be removed from the syringe to get accurate pO2 measurement.

    In low birth weight infant, it can be difficult to obtain blood sample from the umbilical artery, especially if it is small. In such situations, the newborn should be carefully evaluated for arterial academia, since isolated venous blood gas pH can be normal.


  3. 4.

    Umbilical cord blood gas analysis is routinely ordered by obstetricians if there is suspicion of neonatal depression. It reflects the fetal condition immediately before delivery and is a more objective indication of a newborn’s condition than Apgar score, as Apgar score is usually done after the delivery at 1 min, 5 min, and 10 min interval. However, there is usually a time lag between blood gas sampling and analysis. In the meantime neonatal condition should be assessed by the Apgar score.

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Sep 23, 2017 | Posted by in Uncategorized | Comments Off on Blood Gas: Fetal

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