Each time the uterus contracts, blood flow to the placenta decreases and the amount of oxygen the baby receives from the mother decreases. The baby must live off the oxygen already in his blood. Ordinarily, the baby has enough oxygen to carry him through the contraction until he can get more from the mother before the next contraction starts. When the baby has difficulty getting oxygen, changes in the baby's heart rate alert the health-care professionals to take action to help correct the situation. Mothers in labor can be monitored both externally and internally.
External monitoring is virtually painless and generally simple.
To obtain the baby's heart rate and the mother's uterine contractions, two monitor leads are placed on the abdomen outside the uterus by straps. Lubricant may be placed on the abdomen where the baby's heart rate is heard to help pick up the sound. Ultrasonic waves detect heart action and obtain the baby's heart rate. A normal baby's heart rate tracing looks like a choppy, irregular line. An average baby's heart rate is 120 to 160 beats per minute.