A & B spells (Apnea and Bradycardia spells) 
Episodes when the baby stops breathing for at least 15 seconds and the baby’s heart rate slows down to less than 100 beats per minute (normal is around 120-160 beats per minute). Premature babies have apnea and bradycardia spells more often than full-term babies, but babies born full-term can also have them. The cause of these spells is not clearly understood. Some researchers think the spells are related to centers inside the brain that regulate breathing and that may not be fully developed. Touching your baby gently or rocking the incubator slightly will almost always get the baby to start breathing again, which increases the heart rate. Medications (theophylline or caffeine) are often used to treat these spells in newborn babies. 

There are two types of apnea (baby stops breathing): central and obstructive.

Prematurely born babies often have a combination of these two types of apnea.

Apnea does not cause sudden infant death syndrome (SIDS). However, prematurely born babies are more likely to get SIDS because they are prematurely born. No one knows exactly why this is the case.