During normal labor uterine contractions compress uterine blood vessels, resulting in intermittent interruption to blood flow into the placenta. There is no oxygen delivery to the fetus at the height of conctractions with subsequent restoring of oxygen delivery between contractions when the uterus is relaxed. There is only a slight fall in blood pH in a normal labor with a healthy fetus and placenta. On the other hand all abnormalities in the mother, placenta or fetus which prevent such succesful adaptation to the effects of uterine contractions or factors that prolonge labor result in intrapartum fetal hypoxemia.

The fetus is equipped with wide range of adaptive mechanisms so moderate degree of hypoxemia is tolerated for some time. If the condition persists, the switching to anaerobic metabolism results in increasing production of lactate, acidosis and asphyxia. Prolonged periods of low oxygen concentration and acidosis have adverse effects on the fetal circulation, intrauterine fetal breathing, the brain, kidneys and other organs. Unless immediate delivery the result is irreversible organ damage to susceptible organs and ultimately cardiac arrest and intrapartum death.

Causes of birth asphyxia

All the conditions which predispose to intrauterine hypoxia (fetal distress) prior to the onset of contractions may increase the asphyxial stress during labor.

• Maternal  —  maternal shock, cardiac arrest or grand mal fits, preeclampsia, pregnancy cholestasis, severe anemia, smoking, compression of the aorta by the gravid uterus, chronic renal failure and heart disease, administration of anesthetics, sedative and hypotensive drugs to the mother during labor, prolonged gestation, uterine rupture
• Placental  —  diseases of the placenta are frequently an extension of maternal disease. Infarcts, abnormal villous maturation, placenta praevia, placental abruption, widespread vilitis, massive perivillous fibrin deposition, villous oedema, thrombotic occlusion of fetal chorionic vessels
• Umbilical cord  —  abnormal cord lenght, entaglements around fetal body and nuchal cords (only a cord tightly around the neck more than once is a risk factor), true knots, umbilical cord prolaps or compression, funisitis, umbilical vessels thrombosis, velamentous cord insertion, acute fetal blood loss with rupture of vasa previa
• Fetal conditions  —  fetal size (large fetus/small growth-retarded fetus), malpresentation, fetal malformations, chronic fetal anemia and cardiac failure from any cause
• Mechanical factors  —  excessive uterine contractions, prolonged labor, pelvic abnormalities