The temporary, fine hair that sometimes covers a newborn’s body is called lanugo, and its presence can be a surprising sight for new parents. This soft, downy coat develops during gestation and serves a temporary, yet significant, function in the womb. While not all babies are born with lanugo, its presence is a normal part of development, particularly in infants born early. Understanding when this hair appears and naturally sheds can help alleviate concerns about this fleeting feature of infancy.
What Exactly Is Lanugo?
Lanugo is the first type of hair produced by the body’s hair follicles during development. It is characterized by its extremely fine, soft texture and lack of pigment, making it appear light or translucent against the skin. Unlike the hair on an adult’s head, lanugo lacks a central core, or medulla, which contributes to its delicate, feathery nature.
This downy hair typically covers most of the body surface, being most noticeable on the back, shoulders, forehead, and cheeks. It is absent only on parts of the body that do not have hair follicles, such as the palms of the hands and the soles of the feet. Lanugo is distinctly different from the vellus hair, the “peach fuzz” that replaces it, and the thicker terminal hair that grows in childhood and adulthood.
The Typical Timeline of Appearance and Shedding
The appearance of lanugo begins early in the prenatal period, starting around the fourth or fifth month of gestation (16 to 20 weeks). It quickly becomes abundant, reaching its maximum coverage around the 28th week of pregnancy.
For most fetuses, the shedding process begins well before birth, usually between the 33rd and 36th weeks of gestation. As the hairs shed into the amniotic fluid, the developing baby swallows them, and they become a component of meconium, the infant’s first stool. This means that many full-term infants are born without any visible lanugo.
If a baby is born prematurely, it will shed postnatally. For newborns who still have lanugo, it is a temporary feature that typically disappears on its own within the first few weeks or months after delivery. The presence of lanugo at birth is more common in premature babies because they miss the final weeks of in-utero shedding.
The Purpose of Lanugo in Fetal Development
The primary function of lanugo is connected to the protective coating on the fetus known as vernix caseosa. Vernix is a thick, white, waxy substance that covers the fetal skin, shielding it from constant exposure to amniotic fluid.
The fine lanugo hairs serve as an anchor, creating a physical surface that helps the vernix adhere to the skin. Without this fibrous layer, the vernix would not stay in place, compromising its ability to protect the skin from damage or chapping caused by the fluid.
Beyond protection, the combination of lanugo and vernix aids in regulating the fetus’s temperature before significant body fat has developed to provide insulation. Some research also suggests that the movement of the hair in the amniotic fluid stimulates sensory receptors in the skin. This stimulation may play a role in promoting healthy fetal growth and development through the release of certain hormones.
Lanugo Retention and Appearance in Later Life
While the retention of lanugo for a few months after birth is normal, its persistence beyond infancy or its reappearance later in life often signals an underlying medical condition. In these non-infant cases, the hair growth is a symptom, not a condition itself.
One recognized reason for lanugo growth in older children or adults is severe malnutrition, such as that seen in anorexia nervosa. When the body lacks sufficient insulating fat, it attempts to conserve heat by growing this fine, downy hair as a natural response. The lanugo acts as a makeshift layer of insulation, appearing on the face, arms, and torso. In all non-infant cases, the appearance of lanugo typically resolves once the underlying medical issue, such as nutritional deficiency, is successfully addressed.