New parents often wonder why some newborns arrive with a full, dark head of hair while others are completely bald. The difference in hair volume at birth is a completely normal biological variation and is not a reflection of the baby’s future hair growth or health. The answer to this common question lies in a complex interplay of genetic programming, the timing of fetal development, and the influence of maternal hormones during pregnancy.
Fetal Hair Development and Lanugo
Hair growth begins surprisingly early in the womb, with hair follicles forming around the 10th week of gestation. The first coat of hair a fetus grows is called lanugo, a fine, soft, and unpigmented type of hair that develops across the body, including the scalp, around 16 to 20 weeks of pregnancy. This downy hair serves an important function by helping to hold the protective, waxy vernix caseosa onto the skin.
For full-term babies, lanugo is typically shed into the amniotic fluid between 32 and 36 weeks of gestation. If the lanugo is shed before birth, the baby appears bald because the new, finer vellus hair has not yet grown in noticeably. Conversely, babies born with hair have either retained their lanugo until birth or have already begun replacing it with thicker vellus or terminal hair. Premature infants are more likely to be born with a noticeable covering of lanugo because they have not reached the gestational age when this initial coat is normally shed.
The Core Drivers: Genetics and Hormonal Influence
The fundamental blueprint for a baby’s hair density, color, and texture is encoded in their genetics, which dictate how the hair follicles develop and cycle. Every baby is born with all the hair follicles they will ever have, and these follicles are programmed to produce hair according to inherited traits. Researchers have identified multiple genetic markers that contribute to the amount of hair a baby has at birth, and these inherited factors account for much of the variation observed between infants.
Beyond genetics, the powerful surge of maternal hormones during the later stages of pregnancy significantly influences fetal hair growth. High levels of hormones like estrogen cross the placenta and stimulate the hair follicles into an extended growth phase while the baby is in the womb. This hormonal boost encourages the hair to grow longer and thicker than it otherwise might at that stage of development.
For example, studies have shown an association between severe maternal heartburn during pregnancy and a baby born with more hair. This connection exists because the hormones that relax the mother’s esophageal sphincter (causing heartburn) are the same ones that promote fetal hair growth. The amount of hair a baby has at birth, therefore, provides a retrospective glimpse into the hormonal environment of the late prenatal period.
The Newborn Hair Cycle
Regardless of whether a baby is born with hair, the hair cycle undergoes a predictable, hormonally driven reset shortly after birth. This is caused by the sudden withdrawal of the high levels of maternal hormones, particularly estrogen, that were stimulating the hair follicles in utero. This hormonal shift triggers physiological neonatal hair loss, a form of telogen effluvium, where hairs simultaneously enter the resting phase (telogen).
The old hair then begins to shed, typically between two and six months of age, which can be alarming for parents of hairy babies. This hair loss often appears patchy, especially on the back of the head, because friction from sleeping accelerates the shedding of the resting hair. This temporary balding is simply the body clearing out the hormonally stimulated “birth hair” to make way for the new growth cycle.
The new hair that grows back will often be terminal hair, which may differ from the natal hair in both color and texture. For example, a baby born with dark, thick hair may shed it and regrow hair that is fine and blond. This natural cycling ensures that the baby’s hair growth pattern eventually aligns with their own genetic programming.