A fetus or foetus (; plural fetuses, feti,
foetuses, or foeti) is the unborn offspring
of an animal that develops from an embryo.
Following embryonic development the fetal
stage of development takes place.
In human prenatal development, fetal development
begins from the ninth week after fertilisation
(or eleventh week gestational age) and continues
until birth.
Prenatal development is a continuum, with
no clear defining feature distinguishing an
embryo from a fetus.
However, a fetus is characterized by the presence
of all the major body organs, though they
will not yet be fully developed and functional
and some not yet situated in their final anatomical
location.
== Etymology ==
The word fetus (plural fetuses) is related
to the Latin fētus (“offspring”, “bringing
forth”, “hatching of young”) and the
Greek "φυτώ" to plant.
The British, Irish, and Commonwealth spelling
is foetus, which has been in use since at
least 1594.
The spelling with -oe- arose in Late Latin,
in which the distinction between the vowel
sounds -oe- and -e- had been lost.
This spelling is the most common in most Commonwealth
nations, except in the medical literature,
where fetus is used.
The more classical 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 also used historically.
== Development ==
=== 
Weeks 9 to 16 (2 to 3.6 months) ===
In humans, the fetal stage commences at the
beginning of the ninth week, fertilisation
age or eleventh week gestational age.
At the start of the fetal stage, the fetus
is typically about 30 millimetres (1.2 in)
in length from crown-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 the beginning
of development and have minimal operation.
The genitalia of the fetus starts to form
and placenta becomes fully functional during
week 9.At this point in development, uncontrolled
movements and twitches occur as muscles, the
brain, and pathways begin to develop.
=== Weeks 17 to 25 (3.6 to 6.6 months) ===
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 inches) long.
=== Weeks 26 to 38 (6.6 to 8.6 months) ===
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, or fine hair, 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 38th
week after fertilization.
The fetus is considered full-term between
weeks 36 and 40, when it is 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
human fetus.
When fetal size is less than expected, the
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 ==
Fetal viability refers to a point in fetal
development at which the fetus may survive
outside 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 2003–05, 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 is rare for a baby weighing less than 1.1
pounds (0.50 kg) to survive.When such premature
babies are born, the main causes of mortality
are that the respiratory system and the central
nervous system are not completely differentiated.
If given expert postnatal care, some preterm
babies weighing less than 1.1 pounds (0.50
kg) may survive, and are referred to as extremely
low birth weight or immature infants.Preterm
birth 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.
== Circulatory system ==
=== Before birth ===
The heart and blood vessels of the circulatory
system, form relatively early during embryonic
development, but continue to grow and develop
in complexity in 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 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.
Pulmonary resistance is reduced dramatically,
prompting more blood to move into the pulmonary
arteries from the right atrium and ventricle
of 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, 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.
== Immune system ==
The placenta functions as a maternal-fetal
barrier against 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.
== Developmental problems ==
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 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 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 (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.
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.Some
research shows that fetal ultrasounds (including
Doppler, 3D/4D ultrasound and 2D ultrasound)
can have some effects on birth weight and
neurodevelopment.
A particular concern is the possible link
between the widespread use of fetal ultrasounds
over the years, and the huge increase in the
numbers of autism cases.Congenital disorders
are acquired before birth.
Infants with certain congenital heart defects
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.
Other heart birth defects include ventricular
septal defect, pulmonary atresia, and tetralogy
of Fallot.
== 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, developmental neurobiologists argue
that 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 is possible
from the second half of pregnancy: “The
available scientific evidence makes it possible,
even probable, that fetal pain perception
occurs well before late gestation” wrote
KJS Anand in the journal of the IASP.Whether
a fetus has the ability to feel pain and suffering
is part of the abortion debate.
In the United States, for example, pro-life
advocates have proposed legislation that would
require providers of abortions to inform pregnant
women that their fetuses may feel pain during
the procedure and that would require each
person to accept or decline anesthesia for
the fetus.
== Legal and social issues ==
Abortion of a human pregnancy is legal and/or
tolerated in most countries, although with
gestational time limits that normally prohibit
late-term abortions.
== Other animals ==
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 prevalence of convergent evolution to
the fetal stage shows that it is relatively
easy to develop.
It presumably originates from a delay of egg
release, with the eggs being hatched inside
the parent before being laid.
Over time, the robustness of the egg wall
can be decreased until it becomes little more
than a sac.
The fetuses of most mammals are situated similarly
to 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 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
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.
== See also
