Final answer:
By the end of 16-20 weeks gestation, the fetus shows significant development including senses of touch and hearing, bone ossification, muscle growth, the production of blood cells by the bone marrow, and the formation of vernix on the skin.
Step-by-step explanation:
By the end of 16-20 weeks gestation, a fetus has developed senses of touch and hearing, ossification of bones is underway, and there is considerable muscle development. The bone marrow begins producing blood cells, and a protective substance called vernix starts to form on the skin.
Fetal movements become stronger and more coordinated in this period. Visible changes include the development of fetal fingerprints and the elongating body. Internally, the fetus's organ systems, such as the thyroid, liver, pancreas, and kidneys, are already functioning to an extent, and the liver secretes bile while the fetus recirculates amniotic fluid. Hearing has developed sufficiently for the fetus to respond to sounds, and the initial formation of alveoli in the lungs begins, crucial for future breathing outside the womb.
It's a time of significant developmental progress: the fetus increases in both size and function, preparing its bodily systems for life after birth. By the end of week 20, the fetus is more active and starting to take a more defined human shape.