Lanugo: Downy hair on the body of a fetus or newborn baby. Lanugo is the first hair to be produced by the fetal hair follicles, and it usually appears on the fetus at about 5 months of gestation. Lanugo is very fine, soft, and usually unpigmented. Although lanugo is normally shed before birth, around 7 or 8 months of gestation, it is sometimes present at birth. This is not a cause for concern: Lanugo disappears of its own accord within a few days or weeks.