Labor Related Events

Prolonged labor with fetal distress is the most common causes for HIE associated with a 'labor' event at full-term gestation. Fetal distress is defined as an abnormal drop in the absolute fetal heart rate for a prolonged period of time or persistent late decelerations; which are more subtle dips in the heart rate that persist after the contraction has ended. Fetal distress can occur at any time during labor, and its cause may be unknown in many cases. Placental abruption is a serious problem because a portion or the entirety of the placenta pulls away from the wall of the uterus. This can be due to maternal severe high blood pressure or preeclampsia, conditions in the wall of the uterus such as fibroids, abdominal trauma from an accident, or clotting abnormalities of the mother. Abnormal placement of the placenta over the cervix can result in severe maternal-fetal hemorrhage. This is known as a placenta previa.

A prolapsed cord can result in asphyxia if the cord is not immediately returned to the intrauterine space and a cesarean section performed. A uterine rupture is rare but can occur in women with prior uterine surgery or as a result of contractions on a weakened scar. A uterus that does not relax in between contractions for a prolonged period of time can result in reduced blood flow and oxygen to the fetus. Another cause is a sudden and prolonged drop in maternal blood pressure for any reason, commonly seen after epidurals for pain relief.

Premature rupture of membranes and low amniotic fluid can lead to infections or may be the result of an infection, and can lead to a higher risk of birth complications. Intrauterine growth restriction occurs when the fetus' placenta is malfunctioning causing fetal malnutrition and risk of placental malfunction during labor.

Delivery Related Events

Any abnormal presentation besides the head, such as breech, face, or arm presentations can lead to fetal distress. Birth weights larger than normal and shoulder dystocia are related risk factors for difficult delivery or prolonged labor, both of which can cause severe distress. Maternal intravenous sedation within 4 hours of delivery is another reason for fetal hypoxia at birth; opioid reversal medications to stop the respiratory depression are effective in relieving this problem. Complications of delivery such as mid forceps or high vacuum extraction are rare causes of birth asphyxia or brain trauma. Certain pre-existing conditions increase the chance of death from HIE. Such as macrosomia, intrauterine growth restriction, hyaline membrane disease from prematurity, seizure disorders, low blood glucose and low sodium.