Week 22 Of Pregnancy Of Fetus
Week 22 Of Pregnancy Of Fetus
A fetus (also spelled foetus , fœtus , faetus or fætus , see below) is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth. The plural is fetuses .
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.
Etymology and spelling variations
The word fetus is from the Latin fetus , meaning offspring , bringing forth , hatching of young . It has Indo-European roots related to sucking or suckling, from the Aryan prefix bheu- , meaning "To come into being".
Fœtus or foetus is an "incorrectly written" English spelling that has however 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 nevertheless the most common in most English-speaking countries (except in professional literature). The original spelling based on the correct Latin etymology fetus however persists in Canada and the United States. In addition, fetus is now also the standard English spelling throughout the world in medical journals and other technical usage. The spelling "faetus" was used historically.
The only correct English plural is fetuses . The pseudo-Latin plural feti is sometimes seen in English, but this is based on insufficient knowledge of Latin. (In Latin, fētus is fourth declension, and its plural is fētūs .)
Development
Main article: Prenatal development9 weeks of gestation: condition at start of fetal stage
The fetal stage commences at the beginning of the 9th week. At the start of the fetal stage, the fetus is typically about 30 mm (1.2 inches) in length from crown to rump, and weighs about 8 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.The heart, hands, feet, brain and other organs are present, but are only at the beginning of development and have minimal operation.
Fetuses are not capable of feeling pain at the beginning of the fetal stage, and will not be able to feel pain until the third trimester. At this point in development, uncontrolled movements and twitches occur as muscles, the brain and pathways begin to develop.
16 to 25 weeks after fertilization
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. By the end of the fifth month, the fetus is about 20 cm (8 inches).
26 to 40 weeks of gestation
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 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 35 and 40, which means that the fetus is considered sufficiently developed for life outside the uterus. 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.
Variation in growth
See also: Birth weightThere 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 , placental , or fetal .
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.
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 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: Viability (fetal)The lower limit of viability is approximately five months gestational age, and usually later.
There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable. 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. 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, being 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.
Fetal pain
Main article: Fetal painFetal 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." However, there may be an emerging consensus among developmental neurobiologists that the establishment of thalamocortical connections" (at about 26 weeks) is a critical event with regard to fetal perception of pain. 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 suffer is part of the abortion debate. 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.
Circulatory system
Main article: Fetal circulationThe 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.
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.
Some of the blood from the right atrium does not enter the left atrium, but enters the right ventricle and is pum
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