During labor, your doctor will likely monitor the heart rate of your baby. Due to contractions, which temporarily limit the flow of oxygen to the fetus, the baby's heart rate will naturally change during birth. However, any changes that fall outside the normal range -- approximately 110 to 160 beats per minute -- could indicate that the baby is not getting enough oxygen overall, and steps would be taken to correct this [source: Lucile Packard Children's Hospital at Stanford].
For EFM, two belts are wrapped around the mother's abdomen -- one which measures the baby's heart rate and the other that measures contractions. Alternatively, internal monitoring may take place where an electrode is placed on the baby at the part nearest the cervix, which is usually the top of the head. For this type of monitoring, it is necessary for a woman's water to have broken.