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.

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 (plural fetuses) 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 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. The spelling "faetus" was used historically.

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.

Week 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 30 mm 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 may 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.

Week 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. By the end of the fifth month, the fetus is about 20 cm (8 inches).

Week 26 to 40
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 37 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
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, 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


Viability refers to a point in fetal development at which the fetus may survive outside the womb. 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, 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.

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." 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
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.

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 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 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:

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.

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. 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 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.

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.