Lack of oxygen. Prior to labor, the oxygen line supplying baby is tucked in between the body parts in the womb & generally works well or baby wouldn't form right. Labor stresses baby, increases oxygen needs and increases demand on the placenta. If detaching, wrapped around the neck or tied in a loose knot;labor may lead to a cut off o2 and damage to baby may result. Forgetting to start breathing can do the same thing.
Asphyxia. Labor it is a traumatic event for the fetus. Sometimes during labor ombilical cord gets compressed limiting the blood that gets to the baby. Sometimes during labor the blood supply to baby is reduced even more due to abnormal placenta. Asphyxia in labor means bsically baby is not receiving enough oxygen to maintain vital functions in optimal condition.