Final answer:
The age of viability, indicating the fetus's chance of survival outside the womb, is generally considered to be around 24 weeks of gestational age. At this point in fetal development, significant physiological milestones are necessary for survival post-birth have begun to form.
Step-by-step explanation:
The first chance of survival outside the womb, known as the age of viability, is reached at about 24 weeks gestational age. Fetal viability indicates the point in fetal development at which the fetus is likely to be able to survive outside the uterus. Key developmental milestones are achieved during this period, such as bone marrow taking over erythrocyte synthesis, the beginning of myelination of the spinal cord axons, and the production of surfactant in the lungs which is crucial for the fetus to breathe independently after birth. The chance of survival increases significantly as the gestational weeks advance, with more than 90 percent survival rate post 26 weeks given the necessary intensive medical care. The growth and development of the fetus, from the germinal stage to reaching full term, are critical phases that impact the likelihood of its survival outside the uterus.