| http://www.w3.org/ns/prov#value | - From now on, the fetus will respond to a growing range of sounds.[151]Hair begins to grow on the scalp.All skin layers and structures are present, including hair follicles and glands.[152]By 21 to 22 weeks after fertilization, the lungs gain some ability to breathe air.[153] This is considered the age of viability because survival outside the womb becomes possible for some fetuses.[154]
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