Final answer:
The term that best describes infants born three weeks or more before full term is 'Preterm'. Preterm infants are at risk due to less growth time and may require specialized care.
Step-by-step explanation:
Infants that are born three weeks or more before the pregnancy has reached its full term are best described as Preterm. Preterm infants are usually smaller than full-term infants because they have had less time to grow in the uterus. Preterm birth can lead to low birth weight, which increases the risk of death shortly after birth, especially since key systems such as respiratory and nervous systems may not be fully developed. These infants often require specialized medical care to assist with their underdeveloped organ systems immediately after birth.