Final answer:
The 28-week gestational age is often considered the viability threshold for a fetus, as post this period, the fetus has significantly better chances of surviving with medical assistance.
Step-by-step explanation:
The cut-off for gestational age that is generally considered for the viability of a fetus is 28 weeks. This is because, by the 28th week, the fetus has developed to a point where it bears a resemblance to a baby, with distinct features such as eyelashes, eyebrows, and more muscle and bone development. Babies born after this point often have a considerably higher chance of survival, although they may still require medical assistance, particularly for lung development since the lungs are not yet fully mature. Around 24 weeks is also a critical age because, although the survival rate is lower than at 28 weeks, with intensive medical care, some babies can survive if born at this gestational age.
Babies born after 26 weeks have even higher chances of survival, with over 90% surviving when given necessary medical care. However, babies born prior to 28 weeks need considerable medical intervention due to underdeveloped respiratory and nervous systems. As the fetus approaches 38 to 40 weeks, it is considered full-term, with all organ systems developed and the fetus having gained weight and length, ready for birth.