Lanugo is the fine, soft hair that develops on a fetus in the uterus. This unpigmented hair aids in temperature regulation and provides a protective barrier for delicate skin before significant body fat develops. It also anchors the vernix caseosa, a waxy, protective substance, to the skin, shielding the fetus from amniotic fluid.
When Lanugo Typically Disappears
Lanugo begins to form on a fetus between 16 and 20 weeks of gestation, covering most of the body except for areas like the palms, soles, and lips. As pregnancy progresses, the majority of fetuses shed this hair before birth. This shedding process occurs during the third trimester, specifically between 32 and 36 weeks of gestation. For many full-term babies, the lanugo is completely gone by the time they are born.
However, it is common for some full-term newborns to still have traces of lanugo at birth, with about 30% showing some presence of this fine hair. If a baby is born with lanugo, it generally disappears on its own within a few weeks or up to a few months after birth. Its presence at birth is considered a normal variation and does not indicate a medical concern.
How Lanugo is Shed
Lanugo shedding is a natural part of fetal development. As the lanugo detaches from the baby’s skin, it mixes with the surrounding amniotic fluid. The fetus then swallows this fluid as part of its normal development.
The ingested lanugo, along with other substances from the amniotic fluid, contributes to the formation of meconium. Meconium is the baby’s first stool, a thick, dark, sticky substance passed shortly after birth. This process indicates healthy fetal development.
What Influences Lanugo Presence
The presence of lanugo at birth is largely influenced by the baby’s gestational age. Premature babies, those born before 37 weeks, are more likely to have significant amounts of lanugo because they have not completed the in-utero shedding process. Since the shedding occurs in the later stages of pregnancy, an early arrival means less time for the hair to disappear.
While common in preterm infants, some full-term babies may also retain lanugo for a short period after birth. In rare instances, the re-growth or persistent presence of lanugo in older infants or adults can be associated with certain underlying medical conditions, such as severe malnutrition or specific genetic disorders. However, in newborns, lanugo is a temporary and expected finding.