Final answer:
A premature (preterm) infant is one that is born before the completion of 37 weeks of gestation, with such births posing increased risks for health complications. Pre-term infants tend to weigh less and require specialized medical care to enhance survival, especially when born significantly before the 37-week mark.
Step-by-step explanation:
A premature (preterm) infant is defined as an infant born before the completion of 37 weeks of gestation (A). Prematurity is associated with a higher risk for health challenges due to the shorter period for organ development, including essential systems like respiratory and nervous systems.
Pre-term birth significantly impacts infant size and weight due to the reduced time for growth in the uterus. While average birth weight for a full-term infant is about 3.4 kg (7.5 lb), infants born prematurely tend to have lower birth weights. Pre-term infants are also at a higher risk for early mortality, with survival rates improving with each week closer to term, especially after 26 weeks of gestation when survival rates exceed 90 percent given appropriate medical care.
In the context of fetal development, viability of the fetus plays a crucial role, indicating when a fetus can likely survive outside the uterus, with modern medicine increasing the chances of survival for infants born at or after 23 weeks of gestation.