"Extreme prematurity" refers to a situation when a baby is born very early, sometimes without a full development of the body and organs. Full definition
I've already made the decision that I will absolutely not be breastfeeding unless I have a baby that would be more at risk from formula for an actual medical reason (such as extreme prematurity). (skepticalob.com)
The SGA condition, especially combined with extreme prematurity, makes babies more at risk for certain medical conditions, especially neurological problems such as cerebral palsy. (aboutkidshealth.ca)
In the fetal surgery group, one fetus died in utero at 26 weeks after the surgery, and another died from extreme prematurity. (sciencemag.org)