Compression of the vena cava and pelvic veins by the uterus may occur during the third trimester of pregnancy (typically >20 weeks) as a result of the mother lying in a supine position.
Gravid uterus → compression of the abdominal aorta and IVC → impaired venous return and decrease in cardiac output → placental hypoperfusion → fetal hypoxia → deceleration (CTG)
After repositioning the mother in the left lateral position, the fetal heart rate recovers.
In the mother, supine hypotensive syndrome is characterized by tachycardia, dizziness, and nausea, and occasionally causes syncope.