The fetal heart rate decelerates in the following cases: 1. supine hypotension syndrome, the pregnant woman lying on her back increases the uterus compression of the inferior vena cava, which affects the placenta blood supply and oxygen supply causing the fetal heart rate to drop. 2. low amniotic fluid, low amniotic fluid is a manifestation of placental insufficiency, the amount of urine generated by the fetus decreases, the fetal intrauterine activity space is small, and the umbilical cord is easily compressed causing the fetal heart rate to drop. 3. The fetal heart rate can be quickly restored after changing position. 3, acute intrauterine distress, such as umbilical cord prolapse, umbilical cord wrapping around the body, amniotic fluid fecal contamination, placental abruption, etc. 4, inappropriate use of contractions caused by incomplete uterine rupture. 5, abnormal labor, such as contractions too frequent or uncoordinated uterine contractions when the fetal heart rate drops.