Final answer:
The nurse should explain that the baby's purple face with petechiae could be due to the rapid delivery, which can cause small blood vessels to break and lead to petechiae on the exposed parts during birth.
Step-by-step explanation:
The mother's observation of her baby's purple face with petechiae can be concerning, but within the context of a birth, there's an explanation that is more common and less alarming than the others listed. The nurse's response should be based on the understanding that rapid deliveries can sometimes cause petechiae on the neonate's presenting part, which in this case includes the scalp, forehead, and cheeks. Given the 3-hour labor and spontaneous vaginal delivery, this is a plausible explanation for the petechiae.
The formation of petechiae relates to small capillaries that break under stress, such as the pressure experienced during a rapid birth. These small red or purple spots on the skin result from blood leaking out of damaged vessels. They are not indicative of severe bacterial infections in this context, nor are they characteristic of the normal newborn rash or a sign of child abuse.