Fetus
Unborn offspring of viviparous animals, developing from embryo to birth.
A fetus is the unborn offspring of a viviparous animal that develops from an embryo. In human prenatal development, the fetal stage begins from the ninth week after fertilization, which corresponds to the eleventh week of gestational age, and continues until birth. This stage is part of a continuous process of prenatal development, with no single defining feature that clearly separates an embryo from a fetus. However, a fetus is generally characterized by the presence of all major body organs, though these organs are not yet fully developed or functional, and some may not yet be in their final anatomical positions.
The word "fetus" comes from Latin, meaning "offspring" or "bringing forth." The predominant spelling in British, Irish, and Commonwealth English is "foetus," except in medical usage where "fetus" is standard. The -oe- spelling appeared in the late sixteenth century, arising in Late Latin by analogy with classical Latin words like "amoenus."
In non-human animals, the fetal stage occurs in many vertebrates, including most mammals and many fish, as well as some invertebrates such as certain arthropods and onychophorans. In litter-bearing mammals, each fetus is surrounded by placental tissue and lodged along one of two long uteri, unlike the single uterus of humans. The degree of development at birth varies widely: altricial species are born helpless and require extensive parental care, while precocial animals are born with open eyes, hair, and mobility. Primates, except humans, are precocial at birth. Gestation duration in placental mammals ranges from 18 days in jumping mice to 23 months in elephants, with larger land mammals generally requiring longer gestation. Carrying a fetus imposes costs on the mother, including increased food intake and reduced mobility, but allows young to be more developed at birth, potentially reducing parental care needs. Some organisms use the fetal stage to time births to favorable seasons.
In human development, from weeks 9 to 16, the fetus is about 8 grams and the head constitutes nearly half its size. Breathing-like movements stimulate lung development. By weeks 17 to 25, first-time mothers typically feel fetal movements around 21 weeks. From weeks 26 to 38, body fat increases rapidly, lungs are not fully mature, and neural connections develop. Bones are soft and pliable, fingernails reach fingertips, and lanugo h
- field
- Prenatal development
- known_for
- Stage between embryogenesis and birth in viviparous organisms
- etymology
- From Latin fētus 'offspring, bringing forth, hatching of young'
- human_fetal_start
- 9 weeks after fertilization (11 weeks gestational age)
- full_term
- Between weeks 37 and 40 of gestation
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.
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