Fetus

Lore & Background

The word fetus comes from Latin fētus, meaning 'offspring, bringing forth, hatching of young'. The predominant British, Irish, and Commonwealth spelling is foetus, except in medical usage where fetus is preferred. In human development, the fetal stage starts nine weeks after fertilization, when the fetus is about 30 millimeters long and weighs about 8 grams. The head makes up nearly half its size at that point. Breathing-like movements begin to stimulate lung development, though the lungs are not used for oxygen until after birth. By weeks 17 to 25, a first-time pregnant woman typically feels fetal movements at about 21 weeks. By the end of the fifth month, the fetus is about 20 cm long. From weeks 26 to 38, body fat rapidly increases, bones become fully developed but still soft, and fingernails reach the fingertips. The survival rate for babies born at 25–27 weeks is over 90%. The fetus is considered full-term between weeks 37 and 40, and birth occurs around the 38th week after fertilization.

Reader's Guide

The concept of a fetus is central to understanding prenatal development in viviparous animals, including humans. The fetal stage represents a continuum from the embryonic stage, with no clear defining feature distinguishing an embryo from a fetus, though a fetus is generally characterized by the presence of all major body organs. This stage allows young to be more developed at birth, potentially reducing the need for parental care. However, carrying a fetus imposes costs on the mother, including increased food requirements and reduced mobility. Fetal viability—the point at which a fetus may survive outside the womb—has a lower limit of approximately 5¾ months gestational age, with survival rates increasing significantly after that point. Preterm birth is the most common cause of infant mortality, causing nearly 30 percent of neonatal deaths. The study of fetal growth includes classification as small, appropriate, or large for gestational age, with factors affecting growth being maternal, placental, or fetal in origin. The circulatory system of the fetus differs from postnatal circulation because the lungs are not in use; oxygen and nutrients come from the mother through the placenta and umbilical cord.

Did You Know?

• The word fetus comes from Latin fētus, meaning 'offspring, bringing forth, hatching of young'.
• In humans, the fetal stage begins at the ninth week after fertilization.
• The survival rate for babies born at 25–27 weeks of gestation is over 90%.
• Fetal viability has a lower limit of approximately 5¾ months gestational age.