Everything about Foetus totally explained
A
fetus (or
foetus or
fœtus) is a developing
mammal or other
viviparous vertebrate, after the
embryonic stage and before
birth. The plural is
fetuses, or sometimes
feti. The fetal stage of
prenatal development starts when the major structures have formed, and lasts until birth.
In humans, the fetal stage of
prenatal development starts at approximately the beginning of the 9th week after
fertilization, which is equivalent to the start of the eleventh week in "
gestational age."
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.
Fœtus is an English variation on the Latin spelling, and has been in use since at least 1594, according to the
Oxford English Dictionary, which describes "fœtus" as "incorrectly written". The variant
fœtus may have originated with an error by
Saint Isidore of Seville, in AD 620. The preferred spelling in the United States is
fetus, but the variants
foetus and
fœtus persist in other English-speaking countries and in some medical contexts, as well as in some other languages (for example, French).
Human fetus
The fetal stage starts at the beginning of the 9th week following
fertilization, after the
blastocyst,
zygote, and
embryonic stages. The risk of miscarriage decreases sharply at the beginning of the fetal stage. The fetus isn't as sensitive to damage from environmental exposures as the embryo was, though toxic exposures can often cause physiological abnormalities or minor congenital malformation. Fetal growth can be terminated by various factors, including
miscarriage,
feticide committed by a third party, or induced
abortion.
Development
The following describes some of the specific changes in fetal
anatomy and
physiology by fertilization age (for example the time elapsed since fertilization). Obstetricians often use "
gestational age" which, by convention, is measured from 2 weeks earlier than fertilization. For purposes of this article, age is measured from fertilization rather than from two weeks earlier, except as noted.
Condition at start of fetal stage
The fetal stage commences at the beginning of the 9th week after fertilization (for example at the beginning of the 11th week of
gestational age). Breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen. The heart is beating.
The heart, hands, feet, brain and other organs are present, but are not developed sufficiently for the fetus to survive on its own. The fetus is surrounded by
amniotic fluid which offers protection and allows room for movement. The pregnant woman's
placenta and
umbilical cord provide oxygen, nutrients, and allow waste elimination. At this point, complex and generalized movements occur.
Tooth buds appear, the
limbs are long and thin, and
red blood cells are produced in the
liver, however the majority of red blood cells will be made later in gestation (at 21 weeks) by bone marrow. By the end of this stage, the fetus has reached about 15 cm (6 inches).
16th to 25th weeks
The
lanugo covers the entire body. Eyebrows, eyelashes, fingernails, and toenails appear. The fetus has increased muscle development.
Alveoli (air sacs) are forming in lungs. The
nervous system develops enough to control some body functions. The
cochlea are now developed, though the
myelin sheaths in the neural portion of the auditory system will continue to develop until 18 months after birth. The respiratory system has developed to the point where gas exchange is possible. A woman pregnant for the first time (for example a primiparous woman) typically feels fetal movements at about 18-19 weeks, whereas a woman who has already given birth at least two times (for example a multiparous woman) will typically feel movements around 16 weeks. By the end of the fifth month, the fetus is about 20 cm (8 inches).
26th to 38th weeks
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's 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 38th 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.
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. 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
The lower limit of
viability is approximately five months
gestational age, and usually later. According to
The Developing Human:
Viability is defined as the ability of fetuses to survive in the extrauterine environment... There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable or beyond which survival is assured, but experience has shown that it's rare for a baby to survive whose weight is less than 500 gm or whose fertilization age is less than 22 weeks. Even fetuses born between 26 and 28 weeks have difficulty surviving, mainly because 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; they're referred to as extremely low birth weight or immature infants.... Prematurity is one of the most common causes of morbidity and prenatal death.
During the past several decades, neonatal care has improved with advances in medical science, and therefore the point of viability may have moved earlier. As of 2006, the
two youngest children to survive premature birth are thought to be James Elgin Gill (born on
20 May 1987 in
Ottawa,
Canada, at 21 weeks and 5 days gestational age), and Amillia Taylor (born on
24 October 2006 in
Miami,
Florida, at 21 weeks and 6 days gestational age). Both children were born just under 20 weeks from fertilization, or a few days past the midpoint of an average full-term pregnancy. Despite their premature births, both developed into healthy children.
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." Nevertheless, because pain can involve sensory, emotional and cognitive factors, it's "impossible to know" when painful experiences may become possible, even if it's known when thalamocortical connections are established. For example, in the USA legislation has been proposed by
pro-life advocates requiring abortion providers to tell a woman that the fetus may feel pain during the abortion procedure, and that require her to accept or decline anesthesia for the fetus.
Fetal movement
The parts of the fetal brain that control movement won't fully form until late in the second trimester, and the first part of the third trimester. Control of movement is limited at birth, and purposeful voluntary movements develop in the first year after birth.
Locomotor activity begins during the late
embryonic stage, and changes in nature throughout
development. Muscles begin to move as soon as they're
innervated. These first movements are not reflexive, but arise from nerve impulses originating in the
spinal cord. As the
nervous system matures, muscles can move in response to stimuli, though this isn't a voluntary movement.
Quickening is the first maternally discernible fetal movement, which is often felt around the middle of pregnancy. Women who have already given birth have more relaxed uterine muscles that are consequently more sensitive to fetal motion, and for them fetal motion can sometimes be felt as early as 14 weeks.
Complex and generalized movements occur at the beginning of the fetal stage, with movements and startles that involve the whole body. These movements occur as muscles and neural pathways begin to develop Movement of hands, hips and knees have been observed at nine weeks, stretches and yawns at ten weeks, and isolated limb movements beginning shortly thereafter.
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 mother 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's an opening between the right and left atrium (the
foramen ovale), and most of the blood flows through this hole directly into the left atrium from the right atrium, thus bypassing
pulmonary circulation. The continuation of this blood flow is into the left ventricle, and from there it's 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.
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 association curve to oxygen is shifted to the left, meaning that it'll 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
prostaglandins to permit sufficient time for the surgical correction of the anomalies. Conversely, in cases of
patent ductus arteriosus, where the ductus doesn't 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
In the United States, some states have laws that impose strict punishments for those who inflict violence that results in damage to a fetus or the unwanted termination of a pregnancy. The severity of the punishment, and the stage of fetal development where laws start to apply vary from state to state.
Abortion of a fetus is legal in many countries such as Australia, Canada, Mexico, UK and USA. Many of those countries that allow abortion during the fetal stage have gestational time limits, so that
late-term abortions are not normally allowed.
Non-human fetuses
The fetus of most mammals develops similarly to the
Homo sapiens fetus. After the first stages of development, the human embryo reaches a stage very similar to all other vertebrates. The anatomy of the area surrounding a fetus is different in litter-bearing animals compared to humans: each fetus 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 is similar, with animals also having a poorly developed sense of vision and other senses.
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