Fetal Period

The ninth week of development through birth — the longest stage of prenatal life. The developing human is now called a fetus.

What happens

• Growth — the fetus increases enormously in size and weight; most of the weight is gained in the final months.
• Maturation — organs that formed during the embryonic period are refined and become functional: the lungs prepare to breathe, the brain develops rapidly, and the senses begin to work.
• Viability — at a certain point the fetus could, with help, survive outside the womb, and survival odds improve steadily toward full term.

Key idea

The embryonic period builds the parts; the fetal period grows and finishes them — turning a tiny, formed embryo into a newborn ready for life outside.