What is the likely explanation for low fetal heart rate during pushing?

Head compression: Expulsive efforts (pushing) increases compression of the fetal head. There are some dangerous patterns of low heart rate that may lead an obstetrician to intervene in this process.

Head compression.: Typically, the fetal heart rate decreases episodically with contractions during the pushing stage of labor as the fetal head is compressed and rapidly descends in the birth canal. Head compression causes a parasympathetic neural discharge that slows the heart rate as long as the intracranial pressure continues. Other causes: cord compression, placental failure can also occur.