Foetus

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foetus

A fetus (; also spelled foetus , fœtus , faetus , or fætus , see below) is a developing mammal or other viviparous vertebrate after the embryo nic stage and before childbirth|birth .

In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age , which is the 9th week after fertilization .Klossner, N. Jayne (2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth"

Etymology

The word foetus (plural foetuses ) is from the Latin wikt:en:foetus#Latin|foetus (“offspring”, “bringing forth”, “hatching of young”).Harper, Douglas. (2001). . Retrieved 2007-01-20. It has Proto-Indo-European language|Indo-European roots related to sucking or suckling , from the Aryan prefix bheu- , meaning "To come into being".

Fœtus or foetus is the British, Irish and Commonwealth spelling, which has been in use since at least 1594. 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 Saint Isidore of Seville , in AD 620. This spelling is the most common in most Commonwealth nations (except in medical literature, where its use is barred). The etymologically correct 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.New Oxford Dictionary of English. The spelling "faetus" was used historically.American Dictionary of the English Language. Noah Webster. (1828).

The spelling in the Oxford Encyclopedic English Dictionary, Third Edition (1996), page 537, is 'foetus' with 'foetuses' as the plural; 'fetus' (page 514) is given as the 'US variant of foetus.' However, later editions of the OED clarify the etymology behind the Commonwealth spelling.

Development

Weeks 9 to 16

The fetal stage commences at the beginning of the 9th week. At the start of the fetal stage, the fetus is typically about in length from crown to rump, and weighs about 8& nbsp;grams. The head makes up nearly half of the fetus' size. Breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen.Institute of Medicine of the National Academies, (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. (Sixth Edition). Retrieved 2007-03-05.Greenfield, Marjorie. “". Retrieved 2007-01-20.

Fetuses are not capable of feeling pain at the beginning of the fetal stage, and may not be able to feel pain until the third trimester. ( see Fetus#Fetal_pain|Fetal Pain section ) At this point in development, uncontrolled movements and twitches occur as muscles, the brain and pathways begin to develop.Prechtl, Heinz. 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 16 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.Levene, Malcolm et al. (Blackwell 2000), p. 8. Retrieved 2007-03-04. By the end of the fifth month, the fetus is about 20& nbsp;cm (8& nbsp;inches).

Weeks 26 to 38

Variation in growth

There is much variation in the growth of the 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 , placenta l , or fetal .Holden, Chris and MacDonald, Anita. (Elsevier 2000). Retrieved 2007-03-04.

Maternal factors include maternal weight , body mass index , nutritional state, emotional Stress (medicine)|stress , toxin exposure (including tobacco , alcohol , heroin , and other drugs which can also harm the fetus in other ways), and uterus|uterine blood flow.

Placental factors include size, microstructure (densities and architecture), umbilical cord|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.

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. (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 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

Viability refers to a point in fetal development at which the fetus may survive outside the womb. The lower limit of Viability (fetal)|viability is approximately five months gestational age , and usually later.Halamek, Louis. "", NeoReviews , Vol.4 No.6 (2003): "most neonatologists would agree that survival of infants younger than approximately 22 to 23 weeks’ estimated gestational 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. , p. 103 (Saunders 2003). 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. Retrieved on September 2, 2009 It is rare for a baby weighing less than 500 gm to survive.

When such babies are born, the main causes of perinatal mortality is that the respiratory system and the central nervous system are not completely differentiated. 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 . Preterm birth is the most common cause of perinatal mortality, causing almost 30 percent of neonatal deaths. Retrieved on September 2, 2009

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." Two authors of the study published in JAMA did not report their abortion-related activities, which pro-life groups called a conflict of interest; the editor of JAMA responded that JAMA probably would have mentioned those activities if they had been disclosed, but still would have published the study. See Denise Grady, , New York Times (2005-08-26). MSNBC However, there may be an emerging consensus among developmental neurobiology|neurobiologists that the establishment of Human thalamus|thalamocortical connections" (at about 26 weeks) is a critical event with regard to fetal perception of pain.Johnson, Martin and Everitt, Barry. (Blackwell 2000): "The multidimensionality of pain perception, involving sensory, emotional, and cognitive factors may in itself be the basis of conscious, painful experience, but it will remain difficult to attribute this to a fetus at any particular developmental age." Retrieved 2007-02-21. 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.

Whether a fetus has the ability to feel pain and to suffering|suffer is part of the abortion debate .White, R. Frank. "", American Society of Anesthesiologists Newsletter (October 2001). Retrieved 2007-03-10.David, Barry & and Goldberg, Barth. "", Illinois Bar Journal (December 2002). Retrieved 2007-03-10. 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.Weisman, Jonathan. "", Washington Post 2006-12-05. Retrieved 2007-02-06.

Circulatory system

The circulatory system of a human fetus works differently from that of born humans, mainly because the lungs are not in use: the fetus obtains oxygen and nutrients from the woman through the placenta and the umbilical cord .Whitaker, Kent. (Delmar 2001). Retrieved 2007-03-04.

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 (heart)|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.

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 ).

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 (heart)|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:Dudek, Ronald and Fix, James. (Lippincott 2004). Retrieved 2007-03-04.University of Michigan Medical School, . Retrieved 2007-03-04.

Fetal !! Adult
foramen ovale
ductus arteriosus
extra- hepatic portion of the fetal left umbilical vein
intra-hepatic portion of the fetal left umbilical vein (the ductus venosus )
proximal portions of the fetal left and right umbilical arteries
distal portions of the fetal left and right umbilical arteries

In addition to differences in circulation, the developing fetus also employs a different type of oxygen transport protein|transport molecule than adults (adults use hemoglobin|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.

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 prostaglandin s 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.Dalby, JT. (1978). Environmental effects on prenatal development Journal of Pediatric Psychology, 3, 105-109. 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.
smoking and pregnancy|Smoking during pregnancy may also lead to reduced birth weight. Low birth weight is defined as 2500& nbsp;grams (5.5& nbsp;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.

Legal issues

Abortion law|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 abortion#Legal restrictions on later abortion|late-term abortions are not normally allowed .Anika Rahman, Laura Katzive and Stanley K. Henshaw. , International Family Planning Perspectives (Volume 24, Number 2, June 1998).

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