The frequency of placental abruption fatal to the fetus has declined to about 1 in 800 deliveries. It is estimated that 1 in 12 pregnancies will be complicated by trauma. Motor vehicle accidents are the most common cause of blunt trauma to the pregnant woman. The use of seat-belts with shoulder straps is recommended at all times, including while pregnant. Other causes include falls and, unfortunately, assaults, which appear to be increasing in frequency.
Placental abruption is discussed above and usually develops early following trauma.
In the absence of placental abruption fetal injury and death are uncommon. If the placenta is lacerated, fetal blood may hemorrhage into the maternal circulation, a condition termed fetomaternal hemorrhage. The placenta is a highly vascular organ. However, in severe cases, blood flow to and from the placenta may not be enough to keep the fetus alive. Placenta previa occurs when the zygote implants very low in the uterus, in close proximity to the internal cervical opening.