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Lanugo (/la·nu·go/, from Latin lana “wool”) is very fine, soft, and usually unpigmented, downy hair on the body of a fetus or newborn baby. It is the first hair to be produced by the fetal hair follicles, and it usually appears on the fetus at about 5 months of gestation. It is normally shed before birth, around 7 or 8 months of gestation but is often present at birth and disappears on its own within a few days or weeks.
Lanugo hair will invariably be shed by three to four months after birth. It is replaced by hair covering the same surfaces called vellus hair, but this hair is finer and more difficult to see. The more visible hair that continues into adulthood is called terminal hair. This forms in specific areas and is hormone dependant.
In humans 
Fetal development 
During human development, the lanugo grows on fetuses as a normal part of gestation, but is usually shed and replaced by vellus hair at about 33 to 36 weeks of gestational age. As the lanugo is shed from the skin, it is normal for the developing fetus to consume the hair with the fluid, since it drinks from the amniotic fluid and urinates it back into its environment. Subsequently, the lanugo contributes to the newborn baby's meconium. The presence of lanugo in newborns is a sign of premature birth.
It has been hypothesised that the lanugo of human fetuses aids in temperature regulation.
Lanugo can be observed in malnourished patients, including those with eating disorders. When found along with other physical symptoms, for example, lanugo can help a physician make a diagnosis of anorexia nervosa.
It is often found in teratomas (congenital tumours).
In non-human animals 
Fetal whales, like humans and all other primates, also have lanugo.
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