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80-413: A fetus or foetus ( / ˈ f iː t ə s / ; pl. : fetuses , foetuses , rarely feti or foeti ) is the unborn mammalian offspring 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

160-399: A form in which it comprises several distinct parts connected by blood vessels. The parts, called lobes, may number two, three, four, or more. Such placentas are described as bilobed/bilobular/bipartite, trilobed/trilobular/tripartite, and so on. If there is a clearly discernible main lobe and auxiliary lobe, the latter is called a succenturiate placenta . Sometimes the blood vessels connecting

240-529: A relative of the baby. Nepalese think of the placenta as a friend of the baby; the orang Asli and Malay populations in Malay Peninsula regard it as the baby's older sibling. Native Hawaiians believe that the placenta is a part of the baby, and traditionally plant it with a tree that can then grow alongside the child. Various cultures in Indonesia , such as Javanese and Malay, believe that

320-521: Is not used in English ; occasionally the plural feti is used in English by analogy with second-declension Latin nouns ending in -us . 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 . In humans,

400-473: Is placentae , but the form placentas is more common in modern English. The placenta has evolved independently multiple times, probably starting in fish , where it originated multiple times, including the genus Poeciliopsis . Placentation has also evolved in some reptiles . The mammalian placenta evolved more than 100 million years ago and was a critical factor in the explosive diversification of placental mammals. Although all mammalian placentas have

480-431: 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. X-rays are known to have possible adverse effects on the development of the fetus, and the risks need to be weighed against the benefits. Congenital disorders are acquired before birth. Infants with certain congenital heart defects can survive only as long as

560-461: Is critical for embryo survival. The placenta also provides a reservoir of blood for the fetus, delivering blood to it in case of hypotension and vice versa, comparable to a capacitor . Numerous pathologies can affect the placenta. The placenta often plays an important role in various cultures , with many societies conducting rituals regarding its disposal. In the Western world , the placenta

640-402: Is highly susceptible to anomalies in its growth and metabolism, increasing the risk of birth defects . One area of concern is the lifestyle choices made during pregnancy. Diet is especially important in the early stages of development. Studies show that supplementation of the person's diet with folic acid reduces the risk of spina bifida and other neural tube defects. Another dietary concern

720-411: Is increased. The increased diameter and straighter flow path both act to increase maternal blood flow to the placenta. There is relatively high pressure as the maternal blood fills intervillous space through these spiral arteries which bathe the fetal villi in blood, allowing an exchange of gases to take place. In humans and other hemochorial placentals, the maternal blood comes into direct contact with

800-457: Is most often incinerated . Some cultures bury the placenta for various reasons. The Māori of New Zealand traditionally bury the placenta from a newborn child to emphasize the relationship between humans and the earth. Likewise, the Navajo bury the placenta and umbilical cord at a specially chosen site, particularly if the baby dies during birth. In Cambodia and Costa Rica , burial of

880-558: 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 . Fetal growth is often classified as follows: small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (LGA). SGA can result in low birth weight , although premature birth can also result in low birth weight. Low birth weight increases

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960-496: Is possible from the second half of pregnancy . Evidence suggests that the perception of pain in the fetus occurs well before late gestation. Whether a fetus has the ability to feel pain and suffering is part of the abortion debate . In the United States, for example, anti-abortion advocates have proposed legislation that would require providers of abortions to inform pregnant women that their fetuses may feel pain during

1040-572: Is primarily driven by human poaching and habitat destruction , primarily deforestation . Over 70% of mammal species are in the orders Rodentia , Chiroptera , and Eulipotyphla . Mammal classification has been through several revisions since Carl Linnaeus initially defined the class, and at present , no classification system is universally accepted. McKenna & Bell (1997) and Wilson & Reeder (2005) provide useful recent compendiums. Simpson (1945) provides systematics of mammal origins and relationships that had been taught universally until

1120-465: Is rare for a baby weighing less than 500 g (1 lb 2 oz) 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 500 g (1 lb 2 oz) may survive, and are referred to as extremely low birth weight or immature infants . Preterm birth

1200-426: Is the viviparous placental mammals , so named for the temporary organ ( placenta ) used by offspring to draw nutrition from the mother during gestation . Most mammals are intelligent , with some possessing large brains, self-awareness , and tool use . Mammals can communicate and vocalize in several ways, including the production of ultrasound , scent marking , alarm signals , singing , echolocation ; and, in

1280-401: 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. The heart and blood vessels of the circulatory system form relatively early during embryonic development , but continue to grow and develop in complexity in

1360-413: Is traditionally thought to be sterile , but recent research suggests that a resident, non-pathogenic , and diverse population of microorganisms may be present in healthy tissue. However, whether these microbes exist or are clinically important is highly controversial and is the subject of active research. The placenta intermediates the transfer of nutrients between mother and fetus. The perfusion of

1440-404: Is vulnerable to persistent hypoxia or intermittent hypoxia and reoxygenation, which can lead to generation of excessive free radicals . This may contribute to pre-eclampsia and other pregnancy complications . It is proposed that melatonin plays a role as an antioxidant in the placenta. This begins at day 17–22. Placental expulsion begins as a physiological separation from the wall of

1520-496: Is whether breakfast is eaten. Skipping breakfast could lead to extended periods of lower than normal nutrients in the maternal blood, leading to a higher risk of prematurity , or birth defects. Alcohol consumption may increase the risk of the development of fetal alcohol syndrome , a condition leading to intellectual disability in some infants. Smoking during pregnancy may also lead to miscarriages and low birth weight (2,500 grams (5 pounds 8 ounces). Low birth weight

1600-399: The ductus arteriosus , which directs most of this blood away from the lungs (which are not being used for respiration at this point as the fetus is suspended in amniotic fluid ). With the first breath after birth, the system changes suddenly. Pulmonary resistance is reduced dramatically, prompting more blood to move into the pulmonary arteries from the right atrium and ventricle of

1680-473: The Cretaceous . The relationships between these three lineages is contentious, and all three possible hypotheses have been proposed with respect to which group is basal . These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra) and Exafroplacentalia (basal Afrotheria). Boreoeutheria in turn contains two major lineages— Euarchontoglires and Laurasiatheria . Estimates for

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1760-564: The International Union for Conservation of Nature (IUCN) completed a five-year Global Mammal Assessment for its IUCN Red List , which counted 5,488 species. According to research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495, including 96 recently extinct. The word " mammal " is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from

1840-556: The Latin mamma ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes ( echidnas and platypuses ) and therian mammals ( marsupials and placentals ) and all descendants of that ancestor. Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from

1920-606: The biological classification scheme used, are the primates : apes , monkeys , and lemurs ; the Cetartiodactyla : whales and even-toed ungulates ; and the Carnivora which includes cats , dogs , weasels , bears , seals , and allies. According to Mammal Species of the World , 5,416 species were identified in 2006. These were grouped into 1,229  genera , 153  families and 29 orders. In 2008,

2000-428: The cancer testis antigen PAGE4 and expressed in cytotrophoblasts , CSH1 and KISS1 expressed in syncytiotrophoblasts , and PAPPA2 and PRG2 expressed in extravillous trophoblasts. The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium , very early on in pregnancy at about week 4. The outer layer of the late blastocyst, is formed of trophoblasts , cells that form

2080-543: The chorion and allantois . In humans, the placenta averages 22 cm (9 inch) in length and 2–2.5 cm (0.8–1 inch) in thickness, with the center being the thickest, and the edges being the thinnest. It typically weighs approximately 500 grams (just over 1 lb). It has a dark reddish-blue or crimson color. It connects to the fetus by an umbilical cord of approximately 55–60 cm (22–24 inch) in length, which contains two umbilical arteries and one umbilical vein . The umbilical cord inserts into

2160-476: The chorionic plate (has an eccentric attachment). Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells. This results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotyledons . In humans, the placenta usually has a disc shape, but size varies vastly between different mammalian species. The placenta occasionally takes

2240-647: The class Mammalia ( / m ə ˈ m eɪ l i . ə / ). Mammals are characterized by the presence of milk -producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair , and three middle ear bones . These characteristics distinguish them from reptiles and birds , from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Around 6,400 extant species of mammals have been described and divided into 27 orders . The largest orders of mammals, by number of species , are

2320-406: The dominant terrestrial animal group from 66 million years ago to the present. The basic mammalian body type is quadrupedal , with most mammals using four limbs for terrestrial locomotion ; but in some, the limbs are adapted for life at sea , in the air , in trees or underground . The bipeds have adapted to move using only the two lower limbs, while the rear limbs of cetaceans and

2400-418: The foramen ovale is known as the fossa ovalis . The ductus arteriosus normally closes within one or two days of birth, leaving the ligamentum arteriosum , while the umbilical vein and ductus venosus usually closes within two to five days after birth, leaving, respectively, the liver's ligamentum teres and ligamentum venosus . The placenta functions as a maternal-fetal barrier against

2480-401: 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

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2560-428: The rodents , bats , and Eulipotyphla (including hedgehogs , moles and shrews ). The next three are the primates (including humans , monkeys and lemurs ), the even-toed ungulates (including pigs , camels , and whales ), and the Carnivora (including cats , dogs , and seals ). Mammals are the only living members of Synapsida ; this clade , together with Sauropsida (reptiles and birds), constitutes

2640-439: The sea cows are mere internal vestiges . Mammals range in size from the 30–40 millimetres (1.2–1.6 in) bumblebee bat to the 30 metres (98 ft) blue whale —possibly the largest animal to have ever lived. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale . All modern mammals give birth to live young, except the five species of monotremes , which lay eggs. The most species-rich group

2720-399: 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

2800-416: The barrier function of the placenta. The placenta grows throughout pregnancy . Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14 (DM). In preparation for implantation of the blastocyst, the endometrium undergoes decidualization . Spiral arteries in the decidua are remodeled so that they become less convoluted and their diameter

2880-527: The beginning of development and have minimal operation. Uncontrolled movements and twitches occur as muscles , the brain , and pathways begin to develop. 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. By the end of the fifth month, the fetus is about 20 cm (8 in) long. The amount of body fat rapidly increases. Lungs are not fully mature. Neural connections between

2960-492: The case of humans, complex language . Mammals can organize themselves into fission–fusion societies , harems , and hierarchies —but can also be solitary and territorial . Most mammals are polygynous , but some can be monogamous or polyandrous . Domestication of many types of mammals by humans played a major role in the Neolithic Revolution , and resulted in farming replacing hunting and gathering as

3040-812: The clade originating with the last common ancestor of Sinoconodon and living mammals. The earliest-known synapsid satisfying Kemp's definitions is Tikitherium , dated 225 Ma , so the appearance of mammals in this broader sense can be given this Late Triassic date. However, this animal may have actually evolved during the Neogene. As of the early 21st century, molecular studies based on DNA analysis have suggested new relationships among mammal families. Most of these findings have been independently validated by retrotransposon presence/absence data . Classification systems based on molecular studies reveal three major groups or lineages of placental mammals— Afrotheria , Xenarthra and Boreoeutheria —which diverged in

3120-444: The closest thing to an official classification of mammals, despite its known issues. Most mammals, including the six most species-rich orders , belong to the placental group. The three largest orders in numbers of species are Rodentia : mice , rats , porcupines , beavers , capybaras , and other gnawing mammals; Chiroptera : bats; and Eulipotyphla : shrews , moles , and solenodons . The next three biggest orders, depending on

3200-458: The death of the fetus and where this is rarely not resolved it can lead to its formation into a lithopedion . The existence and implications of fetal pain 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." However, developmental neurobiologists argue that

3280-1418: The divergence times between these three placental groups range from 105 to 120 million years ago, depending on the type of DNA used (such as nuclear or mitochondrial ) and varying interpretations of paleogeographic data. Monotremata Marsupialia Xenarthra Afrotheria Glires Euarchonta [REDACTED] Eulipotyphla Chiroptera Pholidota Carnivora [REDACTED] Perissodactyla [REDACTED] Artiodactyla [REDACTED] Monotremata [REDACTED] Paucituberculata [REDACTED] Didelphimorphia [REDACTED] Microbiotheria Notoryctemorphia [REDACTED] Peramelemorphia [REDACTED] Dasyuromorphia [REDACTED] Diprotodontia [REDACTED] Cingulata [REDACTED] Pilosa [REDACTED] Hyracoidea [REDACTED] Sirenia [REDACTED] Proboscidea [REDACTED] Tubulidentata [REDACTED] Macroscelidea [REDACTED] Afrosoricida [REDACTED] Eulipotyphla [REDACTED] Chiroptera [REDACTED] Pholidota [REDACTED] Carnivora [REDACTED] Perissodactyla [REDACTED] Artiodactyla [REDACTED] Scandentia [REDACTED] Lagomorpha [REDACTED] Placenta Mammalian placentas probably first evolved about 150 million to 200 million years ago. The protein syncytin , found in

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3360-555: 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. Other heart birth defects include ventricular septal defect , pulmonary atresia , and tetralogy of Fallot . An abdominal pregnancy can result in

3440-585: The earlier Triassic , despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century. If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to

3520-451: The end of the 20th century. However, since 1945, a large amount of new and more detailed information has gradually been found: The paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics . Though fieldwork and lab work progressively outdated Simpson's classification, it remains

3600-402: The establishment of thalamocortical connections (at about 6 + 1 ⁄ 2 months) is an essential event with regard to fetal perception of pain. Nevertheless, the perception of pain involves sensory, emotional and cognitive factors and it is "impossible to know" when pain is experienced, even if it is known when thalamocortical connections are established. Some authors argue that fetal pain

3680-448: The exception of humans. The duration of gestation in placental mammals varies from 18 days in jumping mice to 23 months in elephants . 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

3760-522: The fetal chorion , though no fluid is exchanged. As the pressure decreases between pulses , the deoxygenated blood flows back through the endometrial veins. Maternal blood flow begins between days 5–12, and is approximately 600–700 ml/min at term. Deoxygenated fetal blood passes through umbilical arteries to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to form chorionic arteries . Chorionic arteries, in turn, branch into cotyledon arteries . In

3840-467: The fetal stage starts nine weeks after fertilization. At this time the fetus is typically about 30 millimetres ( 1 + 1 ⁄ 4  in) 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. The heart, hands, feet, brain, and other organs are present, but are only at

3920-438: The girl skill in digging clams, and expose boys' placentas to ravens to encourage future prophetic visions. In Turkey , the proper disposal of the placenta and umbilical cord is believed to promote devoutness in the child later in life. In Transylvania and Japan , interaction with a disposed placenta is thought to influence the parents' future fertility. Several cultures believe the placenta to be or have been alive, often

4000-414: 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 . Blood from

4080-412: The heart and less to flow through the foramen ovale into the left atrium . The blood from the lungs travels through the pulmonary veins to the left atrium, producing an increase in pressure that pushes the septum primum against the septum secundum , closing the foramen ovale and completing the separation of the newborn's circulatory system into the standard left and right sides. Thereafter,

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4160-749: The human fetus within their mothers. 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

4240-568: The intervillous spaces of the placenta with maternal blood allows the transfer of nutrients and oxygen from the mother to the fetus and the transfer of waste products and carbon dioxide back from the fetus to the maternal blood. Nutrient transfer to the fetus can occur via both active and passive transport . Placental nutrient metabolism was found to play a key role in limiting the transfer of some nutrients. Adverse pregnancy situations, such as those involving maternal diabetes or obesity , can increase or decrease levels of nutrient transporters in

4320-668: The larger Amniota clade. Early synapsids are referred to as " pelycosaurs ." The more advanced therapsids became dominant during the Guadalupian . Mammals originated from cynodonts , an advanced group of therapsids, during the Late Triassic to Early Jurassic . Mammals achieved their modern diversity in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs , and have been

4400-587: The lobes get in the way of fetal presentation during labor , which is called vasa previa . About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal mature placenta. Some 350 of these genes are more specifically expressed in the placenta and fewer than 100 genes are highly placenta specific. The corresponding specific proteins are mainly expressed in trophoblasts and have functions related to pregnancy . Examples of proteins with elevated expression in placenta compared to other organs and tissues are PEG10 and

4480-410: The maternal-fetal barrier. Deterioration in placental functioning, referred to as placental insufficiency , may be related to mother-to-child transmission of some infectious diseases. A very small number of viruses including rubella virus , Zika virus and cytomegalovirus (CMV) can travel across the placental barrier, generally taking advantage of conditions at certain gestational periods as

4560-416: 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. Mammalian A mammal (from Latin mamma  'breast') is a vertebrate animal of

4640-428: The newborn with a carbon copy of the mother's long-term humoral immunity to see the infant through the crucial first months of extrauterine life. IgM antibodies, because of their larger size, cannot cross the placenta, one reason why infections acquired during pregnancy can be particularly hazardous for the fetus. The placenta and fetus may be regarded as a foreign body inside the mother and must be protected from

4720-412: The normal immune response of the mother that would cause it to be rejected . The placenta and fetus are thus treated as sites of immune privilege , with immune tolerance . For this purpose, the placenta uses several mechanisms : However, the placental barrier is not the sole means of evading the immune system, as foreign fetal cells also persist in the maternal circulation, on the other side of

4800-628: The outer barrier of the placenta (the syncytiotrophoblast ) between mother and fetus, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient retrovirus : essentially a virus that helped pave the transition from egg-laying to live-birth . The word placenta comes from the Latin word for a type of cake , from Greek πλακόεντα/πλακοῦντα plakóenta/plakoúnta , accusative of πλακόεις/πλακούς plakóeis/plakoús , "flat, slab-like", with reference to its round, flat appearance in humans. The classical plural

4880-465: The outer layer of the placenta. This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer. The syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblasts, a process that continues throughout placental development. The syncytiotrophoblast contributes to

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4960-428: The placenta develops. CMV and Zika travel from the maternal bloodstream via placental cells to the fetal bloodstream. Beginning as early as 13 weeks of gestation, and increasing linearly, with the largest transfer occurring in the third trimester, IgG antibodies can pass through the human placenta, providing protection to the fetus in utero . This passive immunity lingers for several months after birth, providing

5040-408: The placenta has a spirit and needs to be buried outside the family house. Some Malays would bury the baby's placenta with a pencil (if it is a boy) or a needle and thread (if it is a girl). In some cultures, the placenta is eaten, a practice known as placentophagy . In some eastern cultures, such as China , the dried placenta ( ziheche 紫 河 车 , literally "purple river car") is thought to be

5120-474: The placenta is believed to protect and ensure the health of the baby and the mother. If a mother dies in childbirth, the Aymara of Bolivia bury the placenta in a secret place so that the mother's spirit will not return to claim her baby's life. The placenta is believed by some communities to have power over the lives of the baby or its parents. The Kwakiutl of British Columbia bury girls' placentas to give

5200-401: 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

5280-435: The placenta potentially resulting in overgrowth or restricted growth of the fetus. Waste products excreted from the fetus such as urea , uric acid , and creatinine are transferred to the maternal blood by diffusion across the placenta. The placenta functions as a selective barrier between maternal and fetal cells, preventing maternal blood, proteins and microbes (including bacteria and most viruses ) from crossing

5360-486: The placental barrier. The trophoblast is the outer layer of cells of the blastocyst (see day 9 in Figure, above, showing the initial stages of human embryogenesis). Placental trophoblast cells have a unique genome-wide DNA methylation pattern determined by de novo methyltransferases during embryogenesis . This methylation pattern is principally required to regulate placental development and function, which in turn

5440-535: 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 . The word fetus (plural fetuses or rarely feti ) comes from Latin fētus 'offspring, bringing forth, hatching of young'. The Latin plural fetūs

5520-716: The primary source of food for humans. This led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, and ultimately the development of the first civilizations . Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food ( meat and dairy products ), fur , and leather . Mammals are also hunted and raced for sport, kept as pets and working animals of various types, and are used as model organisms in science. Mammals have been depicted in art since Paleolithic times, and appear in literature, film, mythology, and religion. Decline in numbers and extinction of many mammals

5600-633: The procedure and that would require each person to accept or decline anesthesia for the fetus. Abortion of a human pregnancy is legal and/or tolerated in most countries, although with gestational time limits that normally prohibit late-term abortions . 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

5680-414: 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. Fetal viability refers to a point in fetal development at which the fetus may survive outside

5760-430: The risk of postpartum bleeding may be reduced in women offered active management of the third stage of labour, however there may be adverse effects and more research is necessary. The habit is to cut the cord immediately after birth, but it may be no medical reason to do this; on the contrary, not cutting the cord could sometimes help the baby in its adaptation to extrauterine life , for preterm infants. The placenta

5840-428: The same functions, there are important differences in structure and function in different groups of mammals. For example, human, bovine, equine and canine placentas are very different at both the gross and the microscopic levels. Placentas of these species also differ in their ability to provide maternal immunoglobulins to the fetus. Placental mammals, including humans, have a chorioallantoic placenta that forms from

5920-548: The therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic , this is a reasonable estimate for the appearance of the crown group. T. S. Kemp has provided a more traditional definition: " Synapsids that possess a dentary – squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view,

6000-413: The transmission of microbes . When this is insufficient, mother-to-child transmission of infectious diseases can occur. Maternal IgG antibodies cross the placenta, giving the fetus passive immunity against those diseases for which the mother has antibodies. This transfer of antibodies in humans begins as early as the fifth month (gestational age) and certainly by the sixth month. A developing fetus

6080-405: The umbilical arteries and re-enters the placenta, where carbon dioxide and other waste products from the fetus are taken up and enter the mother'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

6160-494: The upper arms and shoulders. Small breast buds are present in both sexes. Head hair becomes coarse and thicker. Birth is imminent and occurs around the 38th week after fertilization. The fetus is considered full-term between weeks 37 and 40 when it is sufficiently developed for life outside the uterus . It may be 48 to 53 cm (19 to 21 in) in length when born. Control of movement is limited at birth, and purposeful voluntary movements continue to develop until puberty . There

6240-435: The uterus. The period from just after the child is born until just after the placenta is expelled is called the "third stage of labor". Placental expulsion can be managed actively, for example by giving oxytocin via intramuscular injection followed by cord traction to assist in delivering the placenta. Alternatively, it can be managed expectantly, allowing the placenta to be expelled without medical assistance. Blood loss and

6320-502: The villi, these vessels eventually branch to form an extensive arterio-capillary-venous system, bringing the fetal blood extremely close to the maternal blood; but no intermingling of fetal and maternal blood occurs ("placental barrier"). Endothelin and prostanoids cause vasoconstriction in placental arteries, while nitric oxide causes vasodilation . On the other hand, there is no neural vascular regulation, and catecholamines have only little effect. The fetoplacental circulation

6400-517: The womb. The lower limit of viability is approximately 5 + 3 ⁄ 4 months gestational age and is usually later. There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable. According to data from 2003 to 2005, survival rates are 20–35% for babies born at 23 weeks of gestation ( 5 + 3 ⁄ 4 months); 50–70% at 24–25 weeks (6 – 6 + 1 ⁄ 4 months); and >90% at 26–27 weeks ( 6 + 1 ⁄ 2 – 6 + 3 ⁄ 4 months) and over. It

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