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 (foetuses, fœtuses) or, very rarely, foeti.
In humans, the fetal stage of prenatal development begins eight weeks after fertilization, when the major structures and organ systems have formed, until birth.

Etymology and spelling variations
The fetal stage begins eight weeks after fertilization. The fetus is not 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.
Human fetus
See also: Prenatal development
The following timeline describes some of the specific changes in fetal anatomy and physiology by fertilization age (i.e. the time elapsed since fertilization). However, it should be noted that 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, except as noted.
The risk of miscarriage decreases sharply at the beginning of the fetal stage. It may be 48 to 53 cm (19 to 21 inches) in length, when born.

Development
See also: Birth weight
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. 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.

Variation in growth
Five months is currently the lower limit of viability, and viability usually occurs later.

Viability

Main article: Fetal pain Fetal pain
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.

Circulatory system

Main article: Adaptation to extrauterine life Postnatal development
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 association 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

Main article: Fetal rights Non-human fetuses

Embryo
Pregnancy
Child
Superfetation
Neural development
Fetoscopy
Fetal position
Abortion