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State of acid-base state of fetal blood during physiologic labor
Last reviewed: 08.07.2025
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Literature data indicate an undoubted relationship between the condition of the newborn and the acid-base balance of its blood, therefore, when determining the condition of the fetus during labor, the results of the blood test taken from the skin of its head are of decisive importance, and acidosis can be recognized at any stage of labor. The possibility of taking fetal blood to determine the main parameters even before birth is one of the most significant achievements of obstetrics in recent years. The Zaling test is possible only with sufficient dilation of the uterine os, at least 4-5 cm.
In the study, the Zaling test was performed three times during labor in all women in normal labor - in groups 3, 4, and 6. In group 6, blood was collected from the umbilical vein at the time of birth before the first breath. In some cases, the Zaling test could be performed with a shortened cervix in the absence of labor, but the small number of observations did not allow mathematical processing of the data. Blood from the mother's finger to determine the components of the acid-base balance was taken simultaneously with blood sampling from the presenting part of the fetus. The study of the acid-base balance components was performed immediately after sampling the material using the micro-Astrup device with the determination of the following acid-base balance indicators: current pH, base deficit (excess) - BE, buffer bases - BB, standard bicarbonates - SB, and partial pressure of carbon dioxide - pCO 2.
During physiological labor and normal fetal condition, the acid-base balance of its blood is within the normal range. During labor, a decrease in the actual pH of the blood in the second period of labor, an increase in the base deficit, a decrease in the amount of buffer bases and standard bicarbonates, and an increase in the partial pressure of carbon dioxide are observed. All components of the acid-base balance of fetal blood are significantly different in groups 4 and 6. No significant differences were found between the indicators of groups 3 and 4.
The presented data indicate that with a favorable course of labor and physiological state of the fetus during the period of dilation, there are no significant changes in the metabolism of the fetus, and only in the second period of labor does a significant decrease in pH, an increase in base deficiency, a decrease in the amount of alkaline components and an increase in the partial pressure of carbon dioxide occur, indicating the development of compensated metabolic acidosis.
The study of the components of the acid-base balance of the mother's blood during the dynamics of the labor act did not reveal any significant changes indicating the depletion of the blood's buffer capacity. In all the studied groups, the indicators of the acid-base balance of the mother's blood are within physiological limits. Mathematical analysis revealed a reliable decrease in the values of buffer bases, standard bicarbonates and pCO2 in the 6th group compared to the 4th, but these fluctuations do not go beyond physiological limits.
The presented data do not allow us to judge the presence or absence of a connection in changes in the components of the acid-base balance of the blood of the fetus and the mother.