Dictionary > Fetus


n., plural: fetuses
Definition: the developing offspring post-embryonic stage until birth

The science of Embryology and Fetal development has fascinated mankind since time immemorial. After the embryonic stage, the developing young one enters the stage of fetal development. Fetal development encompasses all the developmental changes that take place before birth.

The amniotic fluid is the magic potion that keeps the young one safe and at peace. In contrast to a zygote and an embryo, a fetus is more differentiated and can be recognized as an individual. It is characterized to possess the major organs in contrast to a developing embryo.

The fetal organs though are not yet fully functional and are still undergoing further development. In humans, the embryo is subsequently called a fetus upon reaching the ninth week after conception until the moment of birth. After being born, the offspring is called an infant or a newborn with complete body functions.

Stage of prenatal development
Figure 1: Stage of prenatal development. Image Credit: Very Well.

Fetus Definition

A fetus is defined as a developing mammal inside the mother’s womb before it is born. The fetal stage succeeds the embryonic developmental stage. During pregnancy, the fertilized egg grows and develops into a fetus, a process that typically takes around nine months in humans. Medically, the exact growing stage at which we recognize a fetus is the 9th week after fertilization or the 11th week of pregnancy/gestation. The baby’s development is a complex and intricate process that involves various stages and milestones.

The process of a baby’s development starts as soon as the sperm contributed by the male parent fertilizes the egg of the female parent. Following fertilization, a zygote is formed that transforms into an embryo and attaches to the mother’s uterine lining.

The details of embryonic development are discussed in a comprehensive lesson on Embryology. The embryo undergoes rapid cell division and differentiation. This manifests into recognizable features like limbs, facial features, and internal organs in the next stage referred to as the fetus or fetal stage. By the end of the second trimester, the fetus is fully formed and it spends the third trimester developing further and growing in size.

Biology definition:
A fetus is a developing offspring after the embryonic stage prior to birth. In humans, the young is at nine weeks after fertilization. This fetal period is characterized by the presence of distinct organs, tissues, and systems (e.g., circulatory, nervous, and digestive systems). The fetal stage is characterized by rapid growth, refinement of functional abilities, and maturation of various tissues and structures.

Etymology: Latin fetus (offspring, brood).
Variant: foetus.
Compare: embryo

Some of the important pointers of the fetal stage are:

  1. The fetus is protected and nourished inside the mother’s womb by the placenta. Placenta is a specialized organ that develops during pregnancy. It allows the fetus to receive oxygen, nutrients, and other essential substances from the mother’s blood while filtering out waste products.
  2. During prenatal development, various factors can affect the health and well-being of the fetus, including the mother’s nutrition and lifestyle, exposure to toxins and infections, and genetic factors. Regular prenatal care and medical monitoring are essential to ensure the healthy development of the fetus and prevent complications.
  3. While the fetus is not yet fully formed or independent, it is still a living organism with its unique DNA and potential for life. The debate over the ethics of abortion centers on the question of when the fetus should be considered a human being with rights and protections under the law.
fetus in the uterus
Figure 2: An illustration of a fetus in the uterus of the mother. Image Credit: Stanford Children’s Health.


  • The word fetus finds its origin in the Latin word “fētus” meaning offspring or young one.
  • Different spellings of the same word:
    • Fetus (US, Canada, Australia)
    • Foetus (UK, Ireland, Commonwealth nations)
  • The plural of the fetus (foetus) is fetuses or feti.

Watch this vid about fetus development:

Development in Humans

Since the fetus is a common term for the post-embryonic stage in many mammals, we will be restricted to the idea of human fetus development in this article. Here, we discuss the different characteristic developments in different pregnancy weeks.

While fetal heartbeat can be tracked quite early in gestation (6-8 weeks), more characteristics take time to manifest. They are discussed below.

Since the embryonic stage lasts for the first 8 weeks of development in humans, we discuss fetal development from 9th week onwards.

Also, an important term while calculating the week of development is “last menstrual period” or “last period”. It pertains to the initial day of a woman’s last menstrual cycle before conception and is employed to approximate the gestational age of the fetus. Although it monitors the advancement of pregnancy in the mother, it is not usually linked with the fetus.

Initial developments in the baby
Figure 3: Initial developments in the baby. Image Credit: Kindred Braverly.


  • Weeks 9 to 16 (2 to 3.6 months)

    Beginning of fetal stage: From week-9 after fertilization
    Height of fetus: Nearly 2.3-11.6 cm (week-9 to week-16)
    Weight of fetus: Nearly 2-100 g (week-9 to week-16)
    Distinctive traits:

    • The baby’s head comprises nearly half of the entire fetus’ size.
    • Breathing-like movements (vital for lung development) in the first trimester.
    • Organs present are the baby’s brain, heart, limbs (both hands and feet), kidney, and other organs (under development and with minimal operation).
    • Fingers and baby’s toes become more distinct.
    • Uncontrolled twitches in body muscles (as neural pathways are also under development).
    • Limb movement is unnoticeable for the mother.
    • Thin skin (veins visible).
    • The sex of the fetus can be determined during this period, although external genitalia may not be fully developed until later.

    10-week stage fetus - 3D ultrasound
    Figure 4: A 3D ultrasound can help in checking a baby’s health at around the 10-week stage. Precautions must be taken. Image Credit: London pregnancy.
  • Weeks 17 to 25 (3.6 to 6.6 months)

    Middle of fetal stage: From week-17 to week-25 after fertilization
    Height of fetus: Nearly 13-34.5 cm (week-17 to week-25)
    Weight of fetus: Nearly 170-680 g (week-17 to week-25)
    Distinctive traits:

    • Fetal movements become more noticeable to the mother.
    • Fetal response to outside stimuli (like sound or light).
    • More defined facial features (like eyebrows, eyelashes and head hair, fine hair on the body).
    • Eyes and ears begin to move to their final position on the baby’s face.
    • Skin thickening just begins as more fat accumulates.
    • Baby’s lungs begin to develop surfactant (a substance that helps the air sacs in the lungs inflate and deflate properly in developing lungs).
    • Beginning of a regular sleep-wake cycle.

    Normal fetal eye at 27 weeks
    Figure 5: Normal fetal eye at 27 weeks. Image Credit: Radiology Key.
  • Weeks 26 to 38 (6.6 to 8.6 months)

    Middle of fetal stage: From week-26 to week-38 after fertilization
    Height of fetus: Nearly 35.6-50 cm (week-26 to week-38)
    Weight of fetus: Nearly 770g-3.5kg (week-26 to week-38)
    Distinctive traits:

    • Continuous increase in weight and body fat (for body temperature regulation).
    • Ability to distinguish between light and dark (since baby’s eyelids can open and close).
    • Continuous development of the nervous system and brain.
    • Fully developed lungs.
    • Baby’s bones are soft and pliable.
    • Head-down position (preparation for birth by baby moving through the birth canal).
    • Periods of rest and activity (proper sleep and wake cycles).
    • Small breast buds are present.

    head down and bottom down (breech) fetus
    Figure 6: Normally, the fetus acquires a head-down position by the end of 3rd trimester. Image Credit: The Open University.


  • Variation in growth

The growth of the human fetus can vary greatly, and when the fetal size is less than expected, it is called intrauterine growth restriction (IUGR) or fetal growth restriction. Factors affecting fetal growth can be maternal, placental, or fetal.

    • Maternal factors include weight, BMI, nutrition, stress, toxin exposure, and uterine blood flow.
    • Placental factors include size, microstructure, blood flow, nutrient utilization, and production.
    • Fetal factors include the genome, nutrient production, and hormone output.

Female fetuses tend to weigh less than males at full term. Fetal growth is classified as small, appropriate, or large for gestational age. SGA can result in low birth weight and increase the risk for perinatal mortality and various health problems. SGA may be associated with growth delay or absolute stunting of growth.

Note it!

“Fetal Growth Variations: What Causes Them?”

When we study Obstetrics and Gynecology, we come across several variations in fetal growth. Several genetic, maternal, and environmental factors cause these variations. We list some of the highly studied factors here.

  1. Maternal health conditions: Women’s health conditions such as high blood pressure, gestational diabetes, thyroid disorders, and kidney disease can all affect fetal growth.
  2. Placental problems: As highlighted in Embryology and Blastula lessons, the placenta is very important for the normal growth of the fetus. It plays an irreplaceable role in providing nutrients and oxygen to the fetus. Hence, any problems with the placenta can be detrimental to fetal growth. Two commonly studied problems are:
    • Placental insufficiency: A condition where the placenta is not functioning properly.
    • Placenta previa: A condition where the placenta covers part or the entire cervix.
  3. Genetics: Fetal growth is influenced by both maternal and paternal genes, and some genetic factors may lead to variations in fetal growth.
  4. Multiple gestations: Twin or higher-order pregnancies can lead to variations in fetal growth, with one fetus growing more slowly or quickly than the others.
  5. Lifestyle factors: This is one of the most common factors causing problems in pregnancies in the modern world. With sedentary lifestyles, poor eating habits, and the normalization of different drugs and alcohol, modern pregnancies are challenged more and more by this factor. Maternal lifestyle factors such as smoking, drug use, or alcohol consumption can all affect fetal growth.
  6. Infections: Infections such as cytomegalovirus, rubella, and toxoplasmosis can all affect fetal growth.
  7. Maternal age: Young maternal age (under 20) or advanced maternal age (over 35) can both increase the risk of variations in fetal growth.
  8. Fetal sex: Male fetuses are generally larger than female fetuses, which can lead to variations in growth.
  9. Race and ethnicity: There are differences in fetal growth patterns based on race and ethnicity, with some groups more likely to have smaller or larger babies born than others.

It is important to note that many factors can interact to affect fetal growth, and individual cases can be complex. Regular prenatal care and monitoring can help identify any concerns about fetal growth and ensure appropriate management.

Factors that cause variations in the growth of the fetus
Figure 7: Factors that cause variations in the growth of the fetus. Image Credit: Dr. Harpreet Narang of Biology Online.


Fetal viability is defined as the stage in a pregnancy at which a fetus has developed enough to potentially survive outside the uterus. This is typically between 24-28 weeks gestation. Some characteristic physiological attributes have to be present to determine the correct fetal viability stage. They are:

  • Ability to breathe independently
  • Ability to maintain a stable body temperature
  • Well-developed organs such as the lungs and brain

Although advances in medical technology have improved the survival rates of premature infants, outcomes can vary widely depending on individual circumstances. Healthcare providers and hospitals used various diagnostic tests and assessments to monitor fetal development and assess viability, including ultrasound imaging, amniocentesis, and fetal heart rate monitoring.

Amniocentesis procedure
Figure 8: Amniocentesis procedure. Image Credit: Bruce Blaus.

Some general statistics related to fetal viability are discussed for objectivity below.

  • The lower limit of fetal viability is approximately 5+3⁄4 months gestational age.
  • Survival rates are 20–35% at 23 weeks, 50–70% at 24-25 weeks, and >90% at 26-27 weeks or later.
  • Babies weighing less than 500 g have extremely low survival rates.
  • Respiratory and central nervous systems are not completely differentiated in premature babies and this becomes one of the main reasons for neonatal deaths.
  • Preterm birth causes almost 30% of neonatal deaths.
  • Preterm birth occurs in 5-18% of all deliveries, more common than postmature birth.

Circulatory System

The fetal circulatory system plays a vital role in supplying the developing fetus with oxygen and nutrients during pregnancy. The placenta serves as the connection between the fetus and the mother’s blood supply, enabling the exchange of gases and nutrients.

Blood is circulated through two main vessels: the “umbilical arteries” and the “umbilical vein”. The umbilical vein carries oxygenated blood from the placenta to the fetus whereas the umbilical arteries carry deoxygenated blood back to the placenta. Additionally, shunts or bypasses exist in the fetal circulatory system that allows blood to avoid the lungs, which are not yet fully developed.

Fetal blood cells are different from adult blood cells in several ways. For example, fetal red blood cells contain a different form of hemoglobin, called fetal hemoglobin, which has a higher affinity for oxygen than adult hemoglobin. This allows the fetus to extract oxygen from the mother’s blood more efficiently.

Fetal versus adult hemoglobin
Figure 9: Fetal versus adult hemoglobin. Image Credit: Student Midwife Studygram.
  • Before birth

Some important pointers for progress in the development of a fetus’ circulatory system before birth are:

    • The heart and blood vessels develop early during embryonic development and continue to grow and become more complex in the growing fetus.
    • A functional circulatory system is necessary for mammalian tissues to grow beyond a few cell layers thick since they require an active blood supply.
    • Prenatal circulation of blood differs from postnatal circulation because the lungs are not fully developed and thus are not in use.
    • The fetus obtains oxygen and nutrients from the mother through the placenta and the umbilical cord, and the blood from the placenta is carried to the fetus by the umbilical vein. About half of this quantity goes to fetal ductus venosus and is eventually carried to the inferior vena cava, while the rest goes to the baby’s liver proper.


  • Postnatal development

Some important pointers for the postnatal development of a fetus’ circulatory system are:

    • When a baby is born, its circulatory system changes suddenly.
    • The lungs start working and blood flows into them from the right side of the heart.
    • The blood from the lungs goes to the left side of the heart and increases the pressure.
    • The increased pressure closes a hole between the two sides of the heart.
    • Other connections in the circulatory system also close soon after birth.
fetal blood circulation and newborn blood circulation
Figure 10: Differences between fetal blood circulation and newborn blood circulation. Image Credit: Wikiwand.


Immune System

The fetal immune system is an important part of a developing fetus, protecting it from infections and diseases during prenatal development. The immune system is made up of various cells, organs, and tissues that work together to recognize and fight foreign invaders like bacteria and viruses. In the fetus, the immune system begins to develop around the 10th week of gestation, with the first immune cells appearing in the liver, spleen, and bone marrow. These immune cells, called lymphocytes, play a critical role in recognizing and fighting infections. While the fetal immune system is not fully developed until after birth, it is still able to provide some level of protection to the developing fetus.

Developmental Problems

Examples of developmental problems in fetuses are listed below. Related and relevant causes (mostly related to expecting mothers) are also listed.

  • Neural tube defects

Neural tube defects are birth defects that affect the brain and spinal cord, and some related structures. They occur when the neural tube, which forms the early stages of these structures, fails to close completely during fetal development.

Examples of neural tube defects include spina bifida and anencephaly. Spina bifida occurs due to insufficient maternal intake of folic acid during pregnancy.

Complication of spina bifida
Figure 11: Complication of spina bifida. Image Credit: CDC.
  • Fetal alcohol syndrome (FAS)

Fetal Alcohol Syndrome (FAS) is a condition that can occur in a fetus when a pregnant woman drinks alcohol. FAS can cause physical, behavioral, and cognitive abnormalities in the child. These can include facial abnormalities, growth deficits, intellectual disabilities, and behavioral problems.

Fetal Alcohol Spectrum Disorders awareness infograph
Figure 12: September is celebrated as Fetal Alcohol Spectrum Disorders awareness month. Image Credit: NIAAA-NIH.
  • Low birth weight

Low birth weight is a fetal developmental defect that occurs when an infant weighs less than 2,500 grams (5 pounds 8 ounces) at birth. Infants with low birth weight are at a higher risk of secondary medical problems and may face long-term consequences such as developmental delays and chronic health issues. The causes of low birth weight can vary, but lifestyle choices during pregnancy such as poor maternal nutrition, smoking, and drug use can increase the risk of this developmental problem.

  • Congenital heart defects (CHDs)

Congenital heart defects (CHDs) are structural problems in the heart that are present at birth. They can range from simple conditions that cause no symptoms to complex defects that cause severe, life-threatening symptoms. CHDs occur when the heart is developing during pregnancy, and can result in abnormal blood flow in the heart, which can lead to serious complications if not treated. Some CHDs may require surgery or other medical interventions to correct the defect. Some examples are ventricular septal defect, pulmonary atresia, and tetralogy of Fallot.

Fetal Pain

The topic of fetal pain has been a subject of much debate and controversy in the medical community, particularly in the context of abortion. While some researchers have suggested that fetuses are capable of feeling pain from as early as 20 weeks gestation, others have argued that the neurological development required for pain perception does not occur until much later.

One of the key challenges in studying fetal pain is the lack of direct evidence, as fetuses cannot communicate their subjective experience of pain. However, studies have shown that fetuses respond to noxious stimuli with physiological changes, such as increased heart rate and cortisol levels, which suggest a pain response. Additionally, imaging studies have revealed that the fetal brain is capable of processing sensory information as early as the second trimester.

Despite these findings, the question of whether fetuses truly experience pain remains controversial. A 2010 review by the Royal College of Obstetricians and Gynaecologists concluded that fetal pain perception is unlikely before 24 weeks gestation, due to the incomplete development of the nervous system. (Ref.1) However, a subsequent review published in the Journal of Medical Ethics argued that fetuses may experience pain as early as 12 weeks gestation, based on evidence of fetal brain activity. (Ref.2)

Fetal pain
Figure 13 Fetal pain is a highly debatable topic. Image Credit: The Pitt Pulse.

Other Animals

A fetus is a stage in the prenatal development of viviparous organisms that lies between embryogenesis and birth.

Many vertebrates have fetal stages, including most mammals and many fish, and some invertebrates bear live young.

The fetuses of most mammals are situated similarly to the human fetus within their mothers, but the anatomy of the area surrounding the fetus is different in litter-bearing animals.

The development at birth varies considerably among animals, and even among mammals, with some species being born relatively helpless and requiring considerable parental care and protection, while others are born with open eyes, hair, or down, and are immediately mobile and able to fend for themselves.

The duration of gestation in placental mammals varies widely, from 18 days in jumping mice to 23 months in elephants. The presence of a fetal stage can confer benefits, such as the young being more developed at birth, but also exerts costs on the mother. Since mothers nourish the growing fetus, they must increase their dietary intake. The mother’s physical movement and general comfort are also compromised as the baby grows in size and shape.

long gestation periods in elephants cartoon
Figure 14: A cartoon depicting extremely long gestation periods in elephants (approximately 23 months, i.e., 2 years…). Image Credit: Quora.

Take the Fetus – Biology Quiz!


Choose the best answer. 

1. What is a fetus?
2. Which of the following does not depict a human fetus?
3. Which of these fetal traits is observed at weeks 9 to 16?
4. Fetal age, when weight of the fetus is about 3 kg
5. Which of the following is true about fetal development?

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Further Reading


  • Royal College of Obstetricians and Gynaecologists. (2010). The Care of Women Requesting Induced Abortion. Evidence-based Clinical Guideline Number 7.
  • Derbyshire, S. W. G., & Bockmann, J. (2010). Reconsidering fetal pain. Journal of Medical Ethics, 36(11), 664–668. https://doi.org/10.1136/jme.2010.036293
  • Fetal development: MedlinePlus Medical Encyclopedia”. www.nlm.nih.gov.
  • Levene, Malcolm et al. Essentials of Neonatal Medicine (Blackwell 2000), p. 8.
  • Becher, Julie-Claire. “Insights into Early Fetal Development”, Behind the Medical Headlines (Royal College of Physicians of Edinburgh and Royal College of Physicians and Surgeons of Glasgow October 2004)

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