World Library  
Flag as Inappropriate
Email this Article

Fetal

Article Id: WHEBN0001286038
Reproduction Date:

Title: Fetal  
Author: World Heritage Encyclopedia
Language: English
Subject: Dichloromethane, ICD-9-CM Volume 3, FHT, Impact of health on intelligence, Boonshoft School of Medicine
Collection:
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Fetal

This article is about the stage of human development. For other species, see Fetus (biology). For other uses, see Fetus (disambiguation).

A fetus /ˈftəs/, also spelled foetus, fœtus, faetus, or fætus, is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.

In humans, the fetal stage of prenatal development may be defined as beginning at the 11th week in gestational age, which is the 9th week after fertilization.[1][2] In biological terms, however, prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. The use of the term "fetus" generally implies that a mammalian embryo has developed to the point of being recognizable as belonging to its own species, though the point at which this occurs is subjective. A fetus is also characterized by the presence of all the major body organs, though they may not yet be fully functional, or situated in their final anatomical location.

Etymology

The word fetus (plural fetuses) is from the Latin fētus (“offspring”, “bringing forth”, “hatching of young”).[3]

Fœtus or foetus is the British, Irish and Commonwealth spelling, which has been in use since at least 1594.[4] It arose as a hypercorrection based on an incorrect etymology (i.e. due to insufficient knowledge of Latin) that may have originated with an error by Isidore of Seville, in AD 620.[5][6] This spelling is the most common in most Commonwealth nations (except in the medical literature, where fetus is used). The etymologically accurate original spelling, fetus, is used in Canada and the United States. In addition, fetus is now the standard English spelling throughout the world in medical journals.[7] The spelling "faetus" was used historically.[8]

Development

Main article: Prenatal development

Weeks 9 to 16

In humans, the fetal stage commences at the beginning of the ninth week.[1] At the start of the fetal stage, the fetus is typically about 30 millimetres (1.2 in) in length from crown to rump, and weighs about 8 grams.[1] The head makes up nearly half of the fetus' size.[9] Breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen.[10] The heart, hands, feet, brain and other organs are present, but are only at the beginning of development and have minimal operation.[11][12]

At this point in development, uncontrolled movements and twitches occur as muscles, the brain, and pathways begin to develop.[13]

Weeks 17 to 25

A woman pregnant for the first time (i.e. a primiparous woman) typically feels fetal movements at about 21 weeks, whereas a woman who has already given birth at least two times (i.e. a multiparous woman) will typically feel movements by 20 weeks.[14] By the end of the fifth month, the fetus is about 20 cm (8 inches).

Weeks 26 to 38

The amount of body fat rapidly increases. Lungs are not fully mature. Thalamic brain connections, which mediate sensory input, form. 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 on both sexes. Head hair becomes coarse and thicker. Birth is imminent and occurs around the 40th week. The fetus is considered full-term between weeks 37 and 40,[15] which means that the fetus is considered sufficiently developed for life outside the uterus.[16] It may be 48 to 53 cm (19 to 21 inches) in length, when born. Control of movement is limited at birth, and purposeful voluntary movements develop all the way until puberty.[17][18]

Variation in growth

There is much variation in the growth of the human fetus. When fetal size is less than expected, that condition is known as intrauterine growth restriction (IUGR) also called fetal growth restriction (FGR); factors affecting fetal growth can be maternal, placental, or fetal.[19]

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 fetus genome, nutrient production, and hormone output. Also, female fetuses tend to weigh less than males, at full term.[19]

Fetal growth is often classified as follows: small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (LGA).[20] SGA can result in low birth weight, although premature birth can also result in low birth weight. Low birth weight increases 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.

Viability

Main article: Fetal viability
by gestation.

Viability refers to a point in fetal development at which the human fetus may survive outside the womb. The lower limit of viability is approximately five months gestational age, and usually later.[21]

There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable.[22] According to data years 2003-2005, 20 to 35 percent of babies born at 23 weeks of gestation survive, while 50 to 70 percent of babies born at 24 to 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive.[23] It is rare for a baby weighing less than 500 gm to survive.[22]

When such babies are born, the main causes of perinatal mortality are that the respiratory system and the central nervous system are not completely differentiated.[22] If given expert postnatal care, some fetuses weighing less than 500 gm may survive, and are referred to as extremely low birth weight or immature infants.[22] Preterm birth is the most common cause of perinatal mortality, causing almost 30 percent of neonatal deaths.[24]

Fetal pain

Main article: Fetal pain

Fetal pain, its existence, and its implications are debated politically and academically. According to the conclusions of a review published in 2005, "Evidence regarding the capacity for fetal pain is limited but indicates that fetal perception of pain is unlikely before the third trimester."[25][26] However, developmental neurobiologists that the establishment of thalamocortical connections" (at about 26 weeks) is an essential event with regard to fetal perception of pain.[27] Nevertheless, because pain can involve sensory, emotional and cognitive factors, it is "impossible to know" when painful experiences may become possible, even if it is known when thalamocortical connections are established.[27] Some authors,[28] however, argue that fetal pain is possible from the second half of pregnancy: “The available scientific evidence makes it possible, even probable, that fetal pain perception occurs well before late gestation” wrote KJS Anand in the journal of the IASP.[29]

Whether a fetus has the ability to feel pain and to suffer is part of the abortion debate.[30][31] For example, in the USA legislation has been proposed by pro-life advocates that abortion providers should be required to tell a woman that the fetus may feel pain during the abortion procedure, and require her to accept or decline anesthesia for the fetus.[32]

Circulatory system

Main article: Fetal circulation

The heart and blood vessels which form 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 than the postnatal circulation, mainly because the lungs are not in use. The fetus obtains oxygen and nutrients from the woman through the placenta and the umbilical cord.[33]

Blood from the placenta is carried to the fetus by the umbilical vein. About half of this enters the fetal ductus venosus and is carried to the inferior vena cava, while the other half enters the liver proper from the inferior border of the liver. The branch of the umbilical vein that supplies the right lobe of the liver first joins with the portal vein. The blood then moves to the right atrium of the heart. In the fetus, there is an opening between the right and left atrium (the foramen ovale), and most of the blood flows from the right into the left atrium, thus bypassing pulmonary circulation. The majority of blood flow is into the left ventricle from where it is pumped through the aorta into the body. Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries, and re-enters the placenta, where carbon dioxide and other waste products from the fetus are taken up and enter the woman's circulation.[33]

Some of the blood from the right atrium does not enter the left atrium, but enters the right ventricle and is pumped into the pulmonary artery. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the ductus arteriosus, which directs most of this blood away from the lungs (which aren't being used for respiration at this point as the fetus is suspended in amniotic fluid).[33]

Postnatal development

With the first breath after birth, the system changes suddenly. The pulmonary resistance is dramatically reduced ("pulmo" is from the Latin for "lung"). More blood moves from the right atrium to the right ventricle and into the pulmonary arteries, and less flows through the foramen ovale to the left atrium. The blood from the lungs travels through the pulmonary veins to the left atrium, increasing the pressure there. The decreased right atrial pressure and the increased left atrial pressure pushes the septum primum against the septum secundum, closing the foramen ovale, which now becomes the fossa ovalis. This completes the separation of the circulatory system into two halves, the left and the right.

The ductus arteriosus normally closes off within one or two days of birth, leaving behind the ligamentum arteriosum. The umbilical vein and the ductus venosus closes off within two to five days after birth, leaving behind the ligamentum teres and the ligamentum venosus of the liver respectively.

Differences from the adult circulatory system

Remnants of the fetal circulation can be found in adults:[34][35]

Fetal Adult
foramen ovale fossa ovalis
ductus arteriosus ligamentum arteriosum
extra-hepatic portion of the fetal left umbilical vein ligamentum teres hepatis (the "round ligament of the liver").
intra-hepatic portion of the fetal left umbilical vein (the ductus venosus) ligamentum venosum
proximal portions of the fetal left and right umbilical arteries umbilical branches of the internal iliac arteries
distal portions of the fetal left and right umbilical arteries medial umbilical ligaments (urachus)

In addition to differences in circulation, the developing fetus also employs a different type of oxygen transport molecule than adults (adults use adult hemoglobin). Fetal hemoglobin enhances the fetus' ability to draw oxygen from the placenta. Its dissociation curve to oxygen is shifted to the left, meaning that it will take up oxygen at a lower concentration than adult hemoglobin will. This enables fetal hemoglobin to absorb oxygen from adult hemoglobin in the placenta, which has a lower pressure of oxygen than at the lungs.

Immune system

The placenta functions as a selective maternal-fetal barrier against transmission of microbes. However, insufficiency in this function may still cause mother-to-child transmission of infectious diseases.

Also, maternal IgG antibodies cross the placenta, giving the fetus passive immunity against diseases for which the mother has antibodies. This transfer of antibodies begin as early as the 20th week of gestational age, and certainly by the 24th week.[36]

Developmental problems

Congenital anomalies are anomalies that are acquired before birth. Infants with certain congenital anomalies of the heart can survive only as long as the ductus remains open: in such cases the closure of the ductus can be delayed by the administration of prostaglandins to permit sufficient time for the surgical correction of the anomalies. Conversely, in cases of patent ductus arteriosus, where the ductus does not properly close, drugs that inhibit prostaglandin synthesis can be used to encourage its closure, so that surgery can be avoided.

A developing fetus is highly susceptible to anomalies in its growth and metabolism, increasing the risk of birth defects. One area of concern is the pregnant woman's lifestyle choices made during pregnancy.[37] Diet is especially important in the early stages of development. Studies show that supplementation of the woman's diet with folic acid reduces the risk of spina bifida and other neural tube defects. Another dietary concern is whether the woman eats breakfast. Skipping breakfast could lead to extended periods of lower than normal nutrients in the woman's blood, leading to a higher risk of prematurity, or other birth defects in the fetus. During this time alcohol consumption may increase the risk of the development of Fetal alcohol syndrome, a condition leading to mental retardation in some infants.[38] Smoking during pregnancy may also lead to reduced birth weight. Low birth weight is defined as 2500 grams (5.5 lb). Low birth weight is a concern for medical providers due to the tendency of these infants, described as premature by weight, to have a higher risk of secondary medical problems. Some research shows that fetal ultrasounds (including Doppler, 3D/4D ultrasound, and 2D ultrasound) can have negative effect on birth weight and neurodevelopment.[39]

Legal issues

Abortion of a pregnancy is legal and/or tolerated due to facets of the physician-patient relationship in many countries such as Australia, India, Canada, most European countries, and the United States. Many of those countries that allow abortion during the fetal stage have gestational time limits, so that late-term abortions are not normally allowed.[40]

See also

Preceded by
Embryo
Stages of human development
Fetus
Succeeded by
Infancy

References

External links

  • "Prenatal Image Gallery Index" from The Endowment for Human Development (providing numerous motion pictures of human fetal movement that can be viewed online).
  • "In the Womb," video from National Geographic.
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 



Copyright © World Library Foundation. All rights reserved. eBooks from World eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.