Intrauterine Asphyxia: Clinical Implications for Providers of Intrapartum Care

Jenifer Fahey, CNM, MSN, MPH; Tekoa L. King, CNM, MPH


J Midwifery Womens Health. 2005;50(6):498-506. 

In This Article

Fetal Oxygenation During Labor

Labor can increase the risk for compromised oxygenation in the fetus. Uterine contractions produce transient decreases in blood flow to the placenta, which can lead to interruptions in gas exchange across the placental barrier. Occlusion of one or more of the vessels in the umbilical cord can impede circulation to and from the fetus. During these events, the oxygen content of fetal blood may decrease (hypoxemia), and the carbon dioxide or carbonic acid content of the fetal blood may increase (hypercapnia). Despite the adaptive mechanisms discussed above, these decreases in the oxygen content of fetal blood, if repetitive and/or prolonged, can lead to decreased level of oxygen in fetal tissue (hypoxia) and possibly to an increased concentration of hydrogen ions (respiratory acidemia). If hypoxemia and hypoxia persist, the cellular function in the fetus will switch to anaerobic metabolism, and lactic acid will accumulate (metabolic acidemia). The fact that some degree of hypoxemia and acidemia is normal in healthy fetuses during normal labors presents one of the challenges to successfully identifying those fetuses that are experiencing hypoxemia and acidemia significant enough to cause physiologic damage.

Fetuses have adaptive mechanisms that allow them to compensate if hypoxia occurs. These mechanisms include 1) a decrease in heart rate; 2) a reduction in oxygen consumption secondary to cessation of nonessential functions such as gross body movements; 3) a redistribution of cardiac output to preferentially perfuse organs, such as the heart, brain, and adrenal glands; and 4) a switch to anaerobic cellular metabolism.[10] The degree to which these mechanisms are effective in preventing asphyxia depends on the underlying health of the fetus and the placenta as well as the duration, frequency, and intensity of the hypoxemic event(s). A fetus with already compromised placental function, for example, may experience events that interfere with oxygen exchange during labor contractions that might be well tolerated by a healthy fetus. One of the limitations in preventing intrapartum fetal asphyxia is the limited ability to determine where the threshold for developing asphyxia lies in an individual fetus.


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