Final answer:
Variable FHR decelerations are caused by umbilical cord compression, which impedes the blood flow to the fetus, potentially leading to emergency delivery interventions.
Step-by-step explanation:
Variable fetal heart rate (FHR) decelerations are caused by umbilical cord compression. These decelerations are characterized by abrupt decreases in heart rate that can vary in duration, intensity, and timing with respect to uterine contractions. When the umbilical cord becomes compressed, it impedes the oxygenated blood flow to the fetus, which can be reflected as variable decelerations on a fetal heart rate monitor. The presence of such decelerations can require careful monitoring and potential intervention to ensure the safety and wellbeing of both the mother and the fetus, and in severe cases, can lead to an emergency birth using vacuum, forceps, or a Caesarean section.