Lanugo

Lanugo
Classification and external resources
ICD-10	Q84.2
ICD-9-CM	757.4
DiseasesDB	30821
[]

Lanugo (/ləˈnuːɡoʊ, -ˈnjuː-/; from Latin "wool") is very thin, soft, usually unpigmented, downy hair that is sometimes 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 around 16 weeks of gestation and is abundant by week 20. It is normally shed before birth, around seven or eight months of gestation, but is sometimes present at birth. It disappears on its own within a few weeks.

It is replaced by hair covering the same surfaces, which is called vellus hair. This hair is thinner and more difficult to see. The more visible hair that persists into adulthood is called terminal hair. It forms in specific areas and is hormone-dependent.

During human development, the lanugo grows on fetuses as a normal part of gestation, but it 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 hair to be consumed by the developing fetus, since it drinks from the amniotic fluid and urinates into its environment. As a result, lanugo contributes to the newborn baby's meconium. The presence of lanugo on newborns is not necessarily a sign of premature birth. It is seen on infants born at 39 weeks of gestation, that is, full term.

...
Wikipedia