Lanugo is the fine, downy hair that develops on a fetus in the womb, serving a temporary function during development. While new parents may be surprised to see their baby covered in this soft fuzz, its presence is a normal and expected part of the transition from the prenatal to the postnatal world. Understanding the timing of its disappearance helps reassure parents about this natural stage of their newborn’s development.
Defining Lanugo and Its Role
Lanugo is the first type of hair produced by fetal hair follicles, typically appearing around the fourth or fifth month of pregnancy. This hair is extremely fine, soft, and usually unpigmented, giving it an almost translucent appearance. It covers nearly the entire body of the fetus, with the exception of the palms, soles of the feet, and lips.
The primary function of lanugo is to anchor the vernix caseosa to the skin. The vernix caseosa is a thick, white, waxy coating that protects the fetal skin from the surrounding amniotic fluid. Without the physical anchoring provided by the lanugo, the vernix would simply slide off the fetus’s skin.
By holding the vernix in place, lanugo shields the delicate fetal skin from the drying effects of prolonged exposure to the amniotic fluid. This combination also works to insulate the fetus, helping to regulate body temperature until sufficient fat layers develop.
The Typical Disappearance Timeline
For most babies, lanugo disappearance begins well before birth, during the final weeks of a full-term pregnancy. The hair typically starts to shed around 33 to 36 weeks of gestational age. This timing is a sign of increasing fetal maturity, indicating the skin is developing the ability to protect and regulate itself.
As the lanugo sheds in utero, the fine hairs mix with the amniotic fluid, which the developing fetus naturally swallows. The shed lanugo becomes incorporated into the digestive system contents. This material ultimately contributes to the formation of meconium, the baby’s first stool passed after birth.
Despite this prenatal shedding, about 30% of full-term newborns still have remnants of lanugo present at birth. This lingering hair is most commonly observed on the shoulders, back, forehead, and ears. When present, it usually disappears entirely on its own within the first few weeks of life, generally fading completely by the first month or two.
Lanugo in Premature Births
The timing of lanugo shedding is directly correlated with gestational age, meaning premature infants often have a more noticeable coat of hair at birth. Since the typical shedding process occurs in the final eight weeks of pregnancy, a baby born before 37 weeks will not have completed this transition. The earlier the birth, the more abundant the lanugo is likely to be.
The presence of significant lanugo on a premature baby is a normal finding and reflects their earlier arrival. This observation is often used by medical staff as one factor in estimating a newborn’s gestational age. The hair still serves its protective and insulating purpose for the premature infant.
Even for premature babies, this hair is temporary and will still shed post-birth. The lanugo will gradually fall out as the infant continues to mature outside the womb, without any need for intervention.
The Transition to Vellus Hair
Once the lanugo sheds, it is replaced by vellus hair. Vellus hair is the fine, light-colored, short hair commonly referred to as “peach fuzz” that covers most of the human body during childhood and adulthood. This hair is much finer and less conspicuous than the lanugo it replaces.
Vellus hair plays a role in insulating the body and helping with temperature regulation. It is distinct from terminal hair, which is the thicker, longer, and more pigmented hair found on the scalp, eyebrows, and eyelashes. The transition from lanugo to vellus hair establishes the pattern of fine body hair that will remain through childhood.
This replacement hair type is typically permanent, though some vellus hair will transition into terminal hair later in life. Hormonal changes during puberty, for example, cause vellus hair in areas like the armpits and groin to transform into coarser, darker terminal hair.