Final answer:
The fine, downy hair that covers the fetus is called lanugo. It starts to appear around weeks 16 to 20 of pregnancy and is shed just before birth. Lanugo, along with vernix caseosa, helps to protect the fetus's skin.
Step-by-step explanation:
The fine, downy hair that covers the fetus is known as lanugo. This fine, colorless hair begins to grow on the fetus's face and body around weeks 16 to 20 of pregnancy. By covering most of the body, lanugo plays a significant role in fetal development. It is eventually shed close to the time of birth, usually as the fetus develops head hair that is thicker and coarser.
Along with lanugo, the fetus is also coated with a thick waxy substance called vernix caseosa. Vernix provides protection to the fetus's skin in the amniotic environment by preventing it from becoming chapped or irritated. While lanugo is shed, the vernix will become thicker at first, but most of it will also be gone by the time of birth.