A “deceleration” describes a decrease in the heart rate of the fetus from its baseline heartrate. “Late decelerations” are described as a transient decrease in the fetal heart rate occurring at or after the peak of a uterine contraction. Late decelerations may represent some degree of utero-placental insufficiency. All blood flow in and out of the intravillious space stops briefly during a contraction. A normal fetus with normal reserve (oxygen in its bloodstream, in the blood of the placenta, and in the intravillous space) will probably not notice the tiny drop in total oxygen availability during these contractions. But a fetus that has used up its reserve, or cannot maintain its reserve will, over the course of the contraction, may develop some degree of hypoxia, hypercarbia and acidosis. This otherwise normal fetus will respond by slowing its heart rate, to conserve energy. After the contraction passes and fresh blood resupplies the intravillous space, the hypoxia, hypercarbia and acidosis is eased and the fetal heart rate returns to normal.
What Causes Late Decelerations?
Childbirth can cause physical stress to the fetus, causing late decelerations. The condition most frequently happens at the end of a contraction. The fetal heart monitor will show a drop in the baby’s heartbeat per minute (bpm). An in utero baby usually has a heart rate between 120-160 bpm. While a late deceleration is occurring, the baby’s heart rate may drop to as low as 10 – 30 bpm.
Complications of Late Decelerations?
There is a very short amount of time that a baby can tolerate hypoxic conditions. If the condition is not reversed, brain injury can occur. The time period in which the baby can withstand adverse conditions is usually referred to as the fetal reserve. This period of time varies among babies.