Final answer:
Full-term infants are typically born between 36 and 40 weeks of gestational age and have an average birth weight of 3.4 kg. Premature infants are born before 36 weeks and have a lower birth weight. Fetal viability increases significantly after 26 weeks, and full-term birth is optimal for adequate fetal development.
Step-by-step explanation:
Gestational Age for Full-term and Premature Infants
The typical gestational age range for full-term infants is from 36 to 40 weeks after fertilization. Full-term infants usually have a birth weight ranging from 2.5 to 4.5 kg (5.5 to 10 lb), and the average birth length is about 51 cm (20 inches). In contrast, premature infants are born before the 36-week mark, and as a result, tend to have lower body weights and lengths due to a shorter prenatal growth period. In terms of fetal viability, the fetus has a much higher chance of survival if born after 26 weeks of gestation, often necessitating intensive medical care for those born earlier.
Low birth weight is defined as being under 2.5 kg (5.5 lb) and is closely associated with pre-term births, increasing the risk of postnatal mortality. A full-term pregnancy typically lasts about 38.5 to 40.5 weeks, calculated from the first day of the last menstrual period (LMP). The final weeks of gestation are crucial, as the fetus's organ systems mature to levels suitable for survival outside the uterus. Despite a good chance of survival after 26 weeks, full-term birth is optimal, ensuring adequate fetal development, especially of the respiratory and nervous systems.