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Fetal acid-base status
Last reviewed: 08.07.2025
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Fetal respiration is carried out by gas exchange through the uteroplacental system by means of diffusion. Oxygen is absorbed from the mother's blood and metabolic products are released into it. Acidic and alkaline metabolic products are neutralized by the buffer system of fetal blood. When hypoxia develops, underoxidized metabolic products accumulate in the fetal blood, which are neutralized by the buffer system of fetal blood at the first stages, and when its resources are depleted, they cause pronounced metabolic acidosis, which is expressed in a decrease in the pH value, an increase in the base deficit, a decrease in the amount of buffer and standard bicarbonates, and an increase in the partial pressure of CO 2.
Thus, determination of the respiratory function of the fetus and the acid-base balance (ABB) of its blood are the main and most reliable criteria for identifying fetal pathology.
The Zaling method proposed in 1962 is currently widely used to study the acid-base balance of fetal blood. The essence of the method is to study the acid-base balance of microportions of blood taken from the presenting part of the fetus into a pre-heparinized capillary with sufficient dilation of the cervical os (at least 4 cm). The study is conducted using the micro-Astrup device. If the fetal blood pH is 7.25 or higher, there is no hypoxia, but the results can be considered reliable only in the next 15-30 minutes. A rapid decrease in fetal blood pH is a poor prognostic sign. With increasing signs of acidosis in the fetal blood and in the placenta, enzymes are activated and the content of RNA and DNA decreases, which leads to a decrease in the activity of myofibrils. Consequently, with weak labor, fetal hypoxia is always possible. Active labor management using labor stimulation also leads to a decrease in the fetal blood pH, and, consequently, to its hypoxia.
Determination of the components of the acid-base balance of the fetus's blood is the most reliable indicator of its condition. It should be emphasized, however, that the Zaling test, along with its high diagnostic value, also has a number of disadvantages that make its use difficult or even impossible in some cases. Such disadvantages include the impossibility of conducting the study with an intact fetal bladder, small openings of the cervix, and low attachment of the placenta. In addition, there is a risk of fetal infection with frequent sample production and changes in the acid-base balance when the test material comes into contact with oxygen in the environment. The disadvantages also include the need to use complex diagnostic equipment.
The presence of the shortcomings of the Zaling test served as an incentive to search for methods for determining the condition of the fetus, correlated in significance and devoid of the above shortcomings. The presence of a single maternal-fetal circulation allowed us to assume the possibility of diagnosing the condition of the fetus based on the acid-base balance of the mother's blood. However, the study of the acid-base balance of the blood of a pregnant woman and a woman in labor cannot be considered a reliable criterion for determining the condition of the fetus.
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