Lanugo (/la·nu·go/, from Latin lana "wool") is very fine, soft, and usually unpigmented, downy hair as can be found 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 sometimes present at birth and disappears on its own within a few days or weeks.
Lanugo hair will 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 dependent.
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.
Lanugo functions as an anchor to hold the vernix caseosa on the skin. This combination with vernix caseosa protects the delicate fetal skin from damage from the amniotic fluid. The vernix caseosa also helps to transition the fetus to life outside the womb. It is thought to provide lubrication for birth and it is shown to effect theromoregulation, water loss, and innate immunity. Without the lanugo to anchor the vernix caseosa these functions would be lost.
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 or bulimia nervosa.
It is often found in teratomas (congenital tumours).
Fetal whales, like humans and all other primates, also have lanugo.
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