If you are pregnant and you drive, to-date the only advice you will be given is “Never place the belt over the abdomen, belt should be placed low, across the hips and over upper thighs, under the pregnancy.”  Even when worn “correctly” as low as it can go, the seat belt still crosses over the pregnancy, especially as the pregnancy advances and the baby is lower in the pelvis. Plus, studies have shown that while driving the seat belt tends to move up and rest higher on the abdomen area.
In a frontal collision or emergency braking, the sudden deceleration can cause you as a driver or a passenger to slide down under the lap section of the seat belt, towards the dashboard and footwell this motion is referred to as submarining. The result of submarining is serious abdominal, pelvic and spinal injuries. During pregnancy submarining can cause serious harm to the fetus.
During a crash or sudden braking there is nothing to stop pregnant women from moving forward except the seat belt across the tummy and hence pressure on the baby. Most, if not all, new vehicles are now equipped with pre-tensioning seat belts, which means that in the event of a crash or sudden braking the seat belt will automatically tension at an immense force causing pressure on the tummy.