Lanugo is a downy, unpigmented hair found on fetuses and sometimes on malnourished children and adults. While it is usually shed within the womb, lanugo is sometimes still present on newborn babies, particularly if they are born prematurely. Typically, lanugo on newborns is shed within days or weeks and replaced with vellus hair, which is pale, finer, and less fuzzy in appearance. Vellus hair remains on the body throughout life, and is later joined by darker, coarser terminal hair, especially after puberty.
The body develops all the hair follicles it will ever have in the womb. A hair follicle is a tube-like structure in the skin that produces a strand of hair. The follicle is typically composed of a papilla that projects upward into the tube, small blood vessels called capillaries, a bulb that forms the living part of the hair deep in the skin, a sebaceous gland which releases conditioning oils, an erector and a hair shaft which extends upward from the bulb. The hair shaft is composed of scaly dead cells that are stacked closely and extend out of the follicle to form the visible strands of hair.
Lanugo is the first form of hair created by the hair follicle, usually around the fourteenth week of pregnancy. Usually, the fetus sheds this hair while still in the womb, around the seventh or eighth month of pregnancy. The lanugo then becomes part of the amniotic fluid and may be ingested by the baby. This is perfectly healthy and the baby will later excrete the hair in its meconium, or first stools.
The presence of lanugo after birth usually indicates a premature birth and resembles a colorless fur all over the body, with the exceptions of the labia minora and majora, the lips, the nails, the sides of fingers and toes, the palms of the hands, the bottom of feet, and the penis. The hair is often particularly prevalent across the shoulders, back, and cheeks. Lanugo is always shed, often within days or weeks, and does not require any treatment. The condition is not contagious and not threatening to the baby, though a premature birth can lead to other complications.
Once shed, lanugo never returns in a healthy human. Patients with severe malnutrition, however, may begin to notice a growth of furry, light colored hair on their faces and bodies. Starving patients sometimes begin growing lanugo to insulate the body and keep it warm when the patient has poor circulation and not enough body fat. This is particularly common in patients with anorexia nervosa, an eating disorder that causes patients to obsess over body weight and become emaciated using starvation, excessive exercise, and medications. In this case, the anorexia, rather than the lanugo, is treated through therapy and a dietary regimen to ensure weight gain.