No pattern at all. Time of onset and termination of deceleration are unrelated to time of the onset and termination of the contraction. Abrupt is onset and return to baseline. Frequently preceded or followed by acceleration. Described as severe when below 60 bpms and longer than 60 seconds in duration.
Mechanism is transient umbilical cord compression.
Frequent and complete cord compression can result in progressive oxygen debt. Isolated deceleration of greater than 2 minutes duration. Late in course of severe variable decelerations, prolonged deceleration may occur immediately prior to fetal death. Previously described FHR patterns can occur superimposed upon each other complicating interpretation of EFM strip. Short, abrupt accelerations preceding and immediately following deceleration.