Placental abruption is the cause.

There are many reasons for placental abruption, considering that pregnant women suffer from pregnancy hypertension or vascular diseases caused by chronic kidney disease; long-term absorption of second-hand smoke during pregnancy increased vascular fragility, sudden abdominal impact, sudden decrease in intrauterine pressure such as premature rupture of membranes, fetal umbilical cord too short, excessive traction and so on will cause premature detachment.