The first sign that a baby is not getting enough oxygen between contractions is often a drop in the baby's heart rate after a contraction, called a late deceleration.
It may be the result of a direct blow to the head; of the moving head being rapidly stopped, such as when a person's head hits a windshield in a car accident; or by the sudden deceleration of the head without its striking another object.
Bras specifically designed for sports (as opposed to regular bras designed for fashion) reduced the amount of breast movement and cut down on deceleration forces in the breasts.