Would you be surprised if your newborn came out of the womb with a full head of hair?
Most newborns will have very little to no hair on their heads when they are born. Multiple theories have been proposed as to why some newborns are just hairier than the average baby.
Heartburn, skin color, and a full head of hair
One well known theory is that if you have heartburn while pregnant, your baby will have a lot of hair. This thinking has been part of pregnancy folklore for a while. One study in a small group of predominantly white women found that women reporting more moderate to severe heartburn at 36 weeks gestation were more likely to report having a newborn with an average or above average amount of hair. The opposite was also true in this study- women with no or little heartburn reported having newborns with none or little hair. The authors of the study reasoned that variations in maternal hormones in some pregnant women can affect esophageal sphincter opening, thus contributing to an increased likelihood of heartburn. As a side note, the study found that the small percentage of African American newborns included in the study were more likely to have a lot of hair. Other research supports a link between darker skin color and newborn hair fullness.
Hair growth cycle
Several factors can impact hair growth. This small study highlights the impact of maternal hormones and race/ethnicity on hair growth in newborns. In order to understand both the role of hormones and skin complexion, it is first important to understand the fetal and newborn hair growth cycle.
1) Anagen or growth phase: hair follicles grow on the newborn scalp starting at 10 weeks gestation and continuing until 18 to 20 weeks gestation
2) Catagen phase or short phase: hair follicles curl up at 24 to 28 weeks gestation
3) Telogen or resting phase: hair is shed
It is important to remember that all three phases are occurring at the same time.
So how do these phases apply to the small study we discussed earlier? Well, the presence of heartburn may mean a woman has more estrogen. Increased amounts of maternal hormones such as estrogen increase the number of hair follicles in the anagen or growth phase and decreases hair fall out during the telogen or resting phase. And the impact darker skin color? Newborns with darker skin color are slower to move to the telogen or resting phase where hair falls out.
The number of hair follicles a newborn develops in utero can be different between newborns, but it remains the same after birth. After birth, hair that has developed on the front and sides of the head is entering the telogen phase where it is shed. At this same time, a newborn has hair growth on the back of the head that is entering the anagen or growth phase and will fall out at 8 to 12 weeks after birth.
A newborn’s initial hair is short, fine, and with little color. At 3 to 7 months after birth, “intermediate” scalp hair develops. As the follicle structure develops and expresses color, a full head of mature, colored hair will appear on a child- this is usually around 2 years of age.
There can be several explanations for why a newborn has a full head of hair. Although excessive hair growth can be completely normal, it can also be a sign of drug exposure in utero or genetic conditions like Cornelia de Lange syndrome, Coffin-siris syndrome, leprechaunism, Hurler syndrome, or trisomy 18. It is of particular importance to note any hair fullness localized to certain areas of the newborn’s head as this can also indicate an issue. It is important to remember that hair fullness associated with certain genetic conditions is usually present alongside other signs and symptoms. So, having a newborn with a full head of hair does not mean that there is anything atypical unless other issues are present.