Final answer:
Trauma to the pelvis during the third trimester can cause placental abruption, which can lead to decreased fetal heart rate, maternal bleeding, injury to the fetus, and potentially premature labor.
Step-by-step explanation:
By the third trimester of pregnancy, trauma to the pelvis of the mother can lead to serious complications. A significant concern is the potential for placental abruption, which is when the placenta detaches from the wall of the uterus prematurely, and this can lead to a decrease in the fetal heart rate as the fetus may experience distress. Additionally, trauma can cause maternal bleeding, potential injury to the fetus, and may result in premature labor. It's also important to note that circulatory system changes during pregnancy mean that blood volume increases substantially, which could exacerbate bleeding risks in the event of trauma.