Fetus
Unborn offspring of viviparous animals, 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 and continues until birth. The term is used across many vertebrates, including most mammals and many fish, as well as some invertebrates that bear live young.
- field
- Developmental biology, obstetrics
- known_for
- Stage of prenatal development between embryo and birth
- etymology
- From Latin fētus 'offspring, bringing forth, hatching of young'
- spelling_variants
- Fetus (medical); foetus (British, Irish, Commonwealth)
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 humans, the fetal stage starts nine weeks after fertilization. At that time the fetus is about 30 millimeters long from crown to rump and weighs about 8 grams. The head makes up nearly half its size. Breathing-like movements occur to stimulate lung development, not to obtain oxygen. The heart, hands, feet, brain, and other organs are present but minimally operational. Uncontrolled movements and twitches occur as muscles and neural pathways develop. Fetal viability—the point at which a fetus may survive outside the womb—has a lower limit of approximately 5¾ months gestational age. Survival rates vary: 20–35% at 23 weeks, 50–70% at 24–25 weeks, and over 90% at 26–27 weeks. Preterm birth is the most common cause of infant mortality, causing nearly 30 percent of neonatal deaths.
Reader's Guide
The concept of the fetus is central to understanding prenatal development across viviparous animals. In humans, the fetal stage marks a period of rapid growth and organ maturation, beginning at nine weeks after fertilization and ending at birth. The stage is characterized by the presence of all major body organs, though they are not yet fully developed or functional. Fetal development is a continuum with no clear defining feature distinguishing an embryo from a fetus. Fetal viability has significant medical and ethical implications. The lower limit of viability is approximately 5¾ months gestational age, with survival rates increasing sharply after that point. Preterm birth remains the leading cause of neonatal death, accounting for nearly 30 percent of such deaths. Factors affecting fetal growth include maternal weight, nutritional state, toxin exposure, placental function, and fetal genetics. Growth is classified as small, appropriate, or large for gestational age, with small size increasing risks for perinatal mortality and other health problems. The fetal stage also illustrates evolutionary trade-offs: carrying fetuses requires extra food from the mother and affects her mobility, but results in more developed young that may need less parental care. Some organisms time birth to favorable seasons by adjusting the fetal stage. The comparison of fetal size to fruits is a common practice in pregnancy guidance, with sources like the Australian Department of Health and the Scottish National Health Service providing such references.
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, when the fetus is about 30 mm long and weighs about 8 grams.
- Fetal viability lower limit is approximately 5¾ months gestational age; survival rates exceed 90% at 26–27 weeks.
- Preterm birth causes nearly 30 percent of neonatal deaths and occurs in 5% to 18% of all deliveries.
Frequently Asked Questions
What is a fetus?
A fetus is the developing offspring of a live-bearing (viviparous) animal during the period after the embryonic phase has concluded. In humans, that window stretches from roughly the ninth week post-fertilization all the way through to birth.
When does the fetal stage begin in human development?
In human prenatal development, the fetal period kicks in around the ninth week after fertilization. From that point forward it continues uninterrupted until the baby is delivered.
What's the difference between an embryo and a fetus?
The embryonic phase covers the earliest weeks when the basic body plan is being laid down, while the fetal stage picks up once those foundational structures are established and carries on until birth. In simple terms, the embryo is the 'blueprint-building' phase and the fetus is the 'growing-and-maturing' phase.
Where does the word 'fetus' come from?
The term traces back to the Latin word fētus, which meant 'offspring,' 'bringing forth,' or 'hatching of young.' In British, Irish, and Commonwealth English you will often encounter the alternate spelling 'foetus' instead.
Do only humans have a fetal stage?
No—the concept applies broadly across vertebrates that give live birth, including most mammals and a wide range of fish species. Certain invertebrates that also produce live young can be described in comparable terms.
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