A fetus (alternatively foetus or fœtus) is an embryo in later stages of development, from the third month of pregnancy until birth in humans. Fetus literally means 'young one'.
There is much natural variation in the growth of the fetus. Approximately 40% of the variation in birth weight can be accounted for by genetic factors, whereas 60% can be accounted for by environmental factors. Ultimately, the baby should be able to live up to its term growth potential. Factors affecting fetal growth can be maternal, placental, or fetal.
Maternal factors include maternal size, weight, weight for height , nutritional state , anemia, cigarette smoking, substance abuse, or 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.
Inappropriate growth can result in low birth weight. If the newborn is small for gestational age , he or she will have an increased risk for perinatal mortality (death shortly after birth), asphyxia, hypothermia, polycythemia, hypocalcemia, immune dysfunction , neurologic abnormalities, and other long-term health problems. This can be the result of fetal growth restriction.
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 by the umbilical vein. About half of this enters the 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, then 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 placental 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 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).
See Adaptation to extrauterine life for more details
With the first breath after birth, the system changes suddenly. The pulmonary resistance is dramatically reduced. 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 fosse 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. 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.
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.
Differences to the adult circulatory system
Remnants of the fetal circulation can be found in adults:
- The fetal foramen ovale becomes the adult fosse ovalis.
- The fetal ductus arteriosus becomes the adult ligamentum arteriosum.
- The extra-hepatic portion of the fetal left umbilical vein becomes the adult ligamentum teres hepatis (the "round ligament of the liver").
- The intra-hepatic portion of the fetal left umbilical vein (the ductus venosus) becomes the adult ligamentum venosum .
- The proximal portions of the fetal left and right umbilical arteries become the adult umbilical branches of the internal iliac arteries.
- The distal portions of the fetal left and right umbilical arteries become the adult medial umbilical ligaments.
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.
An unborn child is a child in utero: "a member of the species homo sapiens, at any stage of development, who is carried in the womb," according to legislation which passed the US Senate in March 2004.
Since the 1970s in the United States, a debate has alternately raged or simmered over the "personhood" of the fetus before birth, which has been publicized and has been the spot of media attention thanks to the Laci Peterson case. Arguments regarding the personhood of a fetus are particularly relevant to debates over the legal and moral status of abortions.
See also: Unborn Victims of Violence Act
Etymology and spelling variations
The word fetus comes directly from the Latin fetus meaning "offspring" or "young one". The pseudo-Greek foetus is thus etymologically incorrect. In general, the medical community only permits the spelling fetus, but the spelling foetus persists in the wider community, most notably in Britain, and is accepted in some dictionaries.