A fetus or foetus (; : fetuses or foetuses) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic stage, the fetal stage of development takes place. Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, in general a fetus is characterized by the presence of all the major body organs, though they will not yet be fully developed and functional, and some may not yet be situated in their final anatomical location.
In human prenatal development, fetal development begins from the ninth week after fertilization (which is the eleventh week of gestational age) and continues until the birth of a newborn.
Etymology The word fetus (plural fetuses or rarely, the solecism fetiOxford English Dictionary, 2013, s.v. 'fetus' ) comes from Latin fētus 'offspring, bringing forth, hatching of young'.O.E.D.2nd Ed.2005Harper, Douglas. (2001). Online Etymology Dictionary . Retrieved 2007-01-20. The Latin plural fetūs is not used in English; occasionally the plural feti is used in English by analogy with second-declension Latin nouns.
The predominant British, Irish, and Commonwealth spelling is foetus, except in medical usage, where fetus is preferred. The -oe- spelling is first attested in 1594 and arose in Late Latin by analogy with classical Latin words like amoenus.New Oxford Dictionary of English.
Non-human animals
A fetus is a stage in the prenatal development of viviparous organisms. This stage lies between embryogenesis and birth. Many vertebrates have fetal stages, ranging from most mammals to many fish. In addition, some invertebrates bear live young, including some species of onychophora and many arthropods.
The fetuses of most mammals are situated similarly to the human fetus within their mothers.ZFIN, Pharyngula Period (24–48 h) . Modified from: Kimmel et al., 1995. Developmental Dynamics 203:253–310. Downloaded 5 March 2007. However, the anatomy of the area surrounding a fetus is different in litter-bearing animals compared to humans: each fetus of a litter-bearing animal is surrounded by placental tissue and is lodged along one of two long uteri instead of the single uterus found in a human female.
Development at birth varies considerably among animals, and even among mammals. Altricial species are relatively helpless at birth and require considerable parental care and protection. In contrast, precocial animals are born with open eyes, have hair or down, have large brains, and are immediately mobile and somewhat able to flee from, or defend themselves against, predators. Primates are precocial at birth, with the exception of humans.Lewin, Roger. Human Evolution , page 78 (Blackwell 2004).
The duration of gestation in placental mammals varies from 18 days in jumping mice to 23 months in elephants.Sumich, James and Dudley, Gordon. Laboratory and Field Investigations in Marine Life, page 320 (Jones & Bartlett 2008). Generally speaking, fetuses of larger land mammals require longer gestation periods.
The benefits of a fetal stage means that young are more developed when they are born. Therefore, they may need less parental care and may be better able to fend for themselves. However, carrying fetuses exerts costs on the mother, who must take on extra food to fuel the growth of her offspring, and whose mobility and comfort may be affected (especially toward the end of the fetal stage).
In some instances, the presence of a fetal stage may allow organisms to time the birth of their offspring to a favorable season.
Development in humans
Weeks 9 to 16 (2 to 3.6 months)
In humans, the fetal stage starts nine weeks after fertilization.Klossner, N. Jayne, Introductory Maternity Nursing (2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth" At this time the fetus is typically about in length from crown to rump, and weighs about 8 grams. The head makes up nearly half of the size of the fetus. Breathing-like movements of the fetus are necessary for the stimulation of lung development, rather than for obtaining oxygen.Institute of Medicine of the National Academies, Preterm Birth: Causes, Consequences, and Prevention (2006), page 317. Retrieved 2008-03-12 The heart, hands, feet, brain, and other organs are present, but are only at the beginning of development and have minimal operation.The Columbia Encyclopedia (Sixth Edition). Retrieved 2007-03-05.Greenfield, Marjorie. "Dr. Spock.com ". Retrieved 2007-01-20. Uncontrolled movements and twitches occur as muscles, the brain, and pathways begin to develop.Prechtl, Heinz. "Prenatal and Early Postnatal Development of Human Motor Behavior" in Handbook of brain and behaviour in human development, Kalverboer and Gramsbergen eds., pp. 415–418 (2001 Kluwer Academic Publishers): "The first movements to occur are sideward bendings of the head. ... At 9–10 weeks postmestrual age complex and generalized movements occur. These are the so-called general movements (Prechtl et al., 1979) and the startles. Both include the whole body, but the general movements are slower and have a complex sequence of involved body parts, while the startle is a quick, phasic movement of all limbs and trunk and neck."
Weeks 17 to 25 (3.6 to 6.6 months) A woman pregnant for the first time (nulliparous) typically feels fetal movements at about 21 weeks, whereas a woman who has given birth before will typically feel movements by 20 weeks.Levene, Malcolm et al. Essentials of Neonatal Medicine (Blackwell 2000), p. 8. Retrieved 2007-03-04. By the end of the fifth month, the fetus is about long.
Weeks 26 to 38 (6.6 to 8.6 months) The amount of body fat rapidly increases. Lungs are not fully mature. Neural connections between the sensory cortex and thalamus develop as early as 24 weeks of gestational age, but the first evidence of their function does not occur until around 30 weeks. Bones are fully developed but are still soft and pliable. Iron, calcium, and phosphorus become more abundant. Fingernails reach the end of the fingertips. The lanugo, or fine hair, begins to disappear until it is gone except on the upper arms and shoulders. Small breast buds are present in both sexes. Head hair becomes coarse and thicker. The survival rate rises to over 90% for babies born at 25–27 weeks weeks of gestation ( – months).March of Dimes – Neonatal Death , retrieved September 2, 2009. The fetus is considered full-term between weeks 37 and 40. Birth is imminent and occurs around the 38th week after fertilization, followed by life outside the uterus. It may be in length when born. Control of movement is limited at birth, and purposeful voluntary movements continue to develop until puberty.Stanley, Fiona et al. "Cerebral Palsies: Epidemiology and Causal Pathways", page 48 (2000 Cambridge University Press): "Motor competence at birth is limited in the human neonate. The voluntary control of movement develops and matures during a prolonged period up to puberty...."Becher, Julie-Claire. , Behind the Medical Headlines (Royal College of Physicians of Edinburgh and Royal College of Physicians and Surgeons of Glasgow October 2004)
Variation in growth
There is much variation in the growth of the human fetus. When the fetal size is less than expected, the condition is known as intrauterine growth restriction also called fetal growth restriction; factors affecting fetal growth can be maternal, placental, or fetal.Holden, Chris and MacDonald, Anita. Nutrition and Child Health (Elsevier 2000). Retrieved 2007-03-04.
Maternal factors include maternal weight, body mass index, nutritional state, emotional stress, toxin exposure (including tobacco, alcohol, heroin, and other drugs which can also harm the fetus in other ways), and uterine blood flow.
Placental factors include size, microstructure (densities and architecture), umbilical blood flow, transporters and binding proteins, nutrient utilization, and nutrient production.
Fetal factors include the fetal genome, nutrient production, and hormone output. Also, female fetuses tend to weigh less than males, at full term.
Fetal growth is often classified as follows: small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (LGA).Queenan, John. Management of High-Risk Pregnancy (Blackwell 1999). Retrieved 2007-03-04. SGA can result in low birth weight, although premature birth can also result in low birth weight. Low birth weight increases the risk for perinatal mortality (death shortly after birth), asphyxia, hypothermia, polycythemia, hypocalcemia, immune dysfunction, neurologic abnormalities, and other long-term health problems. SGA may be associated with growth delay, or it may instead be associated with absolute stunting of growth.
Fruit size comparison During pregnancy, comparison of the size of the developing fetus to a fruit is a common practice. The fruits used as a reference vary by source. One source, the Australian Department of Health describes an 8-week embryo as the size of a raspberry, and a 21-week old fetus as the size of a rockmelon. The Scottish National Health Service describes an embryo at 5 weeks of gestation as the size of an "apple pip" and a raisin at week 8. Nurses report these comparisons are accurate after measuring expected sizes with actual fruits.
Viability
Fetal viability refers to a point in fetal development at which the fetus may survive outside the womb. The lower limit of viability is approximately months gestational age and is usually later.Halamek, Louis. "Prenatal Consultation at the Limits of Viability ", NeoReviews, Vol.4 No.6 (2003): "most neonatologists would agree that survival of infants younger than approximately 22 to 23 weeks' estimated gestational age [i.e. 20 to 21 weeks' estimated fertilization age] is universally dismal and that resuscitative efforts should not be undertaken when a neonate is born at this point in pregnancy."
There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable.Moore, Keith and Persaud, T. The Developing Human: Clinically Oriented Embryology, p. 103 (Saunders 2003). According to data from 2003 to 2005, survival rates are 20–35% for babies born at 23 weeks of gestation ( months); 50–70% at 24–25 weeks (6 – months); and >90% at 26–27 weeks ( – months) and over.March of Dimes – Neonatal Death , retrieved September 2, 2009. It is rare for a baby weighing less than to survive.
When such premature babies are born, the main causes of mortality are that neither the respiratory system nor the central nervous system are completely differentiated. If given expert postnatal care, some preterm babies weighing less than may survive, and are referred to as extremely low birth weight or immature infants.
Preterm birth is the most common cause of infant mortality, causing almost 30 percent of neonatal deaths. At an occurrence rate of 5% to 18% of all deliveries, it is also more common than postmature birth, which occurs in 3% to 12% of pregnancies.
Circulatory system
Before birth
The heart and blood vessels of the circulatory system form relatively early during embryonic development, but continue to grow and develop in complexity in the growing fetus. A functional circulatory system is a biological necessity since mammalian tissues can not grow more than a few cell layers thick without an active blood supply. The prenatal circulation of blood is different from postnatal circulation, mainly because the lungs are not in use. The fetus obtains oxygen and nutrients from the mother through the placenta and the umbilical cord.Whitaker, Kent (2001). Comprehensive Perinatal and Pediatric Respiratory Care. Delmar. Retrieved 2007-03-04.