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.
In humans, the fetal stage of prenatal development begins about eight weeks after fertilization, when the major structures and organ systems have formed, until birth.
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). 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.
8 to 15 weeks The fetal stage commences at eight weeks when the fetus is typically about 30 mm (1.2 inches) in length from crown to rump and the head makes up nearly half of the fetus' size.. The fetus cannot feel pain, is not yet sentient, and moves involuntarily as tissues, organs and pathways begin to develop. The movements include motor patterns, and localized movement of the arms and legs, hiccups, stretches and yawns, sideward bendings of the head, and generalized movements that involve the whole body. These movements are involuntary, and the parts of the fetal brain that control movement will not fully form until late in the second trimester, and the first part of the third trimester. At this stage, the heart is beating but not functional.The hands, feet, head, brain, and other organs are present, but not yet functional. The breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen. At nine weeks the fetus' involuntary movements include curling toes to move away from an object, and fingers are structurally able to bend. During weeks 9-12, the face is “well-formed,” though the fetal head is only one to three inches long. From weeks 9 to 12, the fetal eyelids close and remain closed for several months, and the appearance of the genitals in males and females becomes more apparent. 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. A fine hair called lanugo develops on the head. The gastrointestinal tract, still forming, starts to collect sloughed skin and lanugo, as well as hepatic products, forming meconium (stool). Fetal skin is almost transparent. More muscle tissue and bones have developed, and the bones become harder. The first measurable signs of EEG movement occur in the 12th week. By the end of this stage, the fetus has reached about 15 cm (6 inches). 16 to 25 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. The quickening, the first maternally discernable fetal movements, are often felt during this period. A woman pregnant for the first time (i.e. a primiparous woman) typically feels fetal movements at about 18-19 weeks, whereas a woman who has already given birth at least two times (i.e. 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). 26 to 38 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 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. 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.