Hair, Skin, and Nails
The development of hair and its different types serve very specific purposes during fetal growth and development, and also reveal information about the baby's health.
Lanugo helps bind a sticky substance known as vernix to the skin and to keep the fetus protected from amniotic fluid; it also acts as a trigger for fetal growth at different stages of pregnancy. Lanugo is usually shed before birth, although some babies may be born with it. Permanent, coarser hairs replace these fine hairs.
The fetal skin has functions and abilities during pregnancy unmatched in adult skin, to include perfect wound healing early in pregnancy; its function changes dramatically in the second half of pregnancy and even affects amniotic fluid levels.
Although fingernail development is slightly ahead of toenails (similar to limb development), both types will reach the tips of the fingers and toes by birth, and some newborns may need their nails clipped – especially those born after 40 weeks.
Background
Fetal skin and hair develop in very specific ways during gestation that allow for protection of skin from amniotic fluid and other contaminants, temperature regulation, and accelerated growth at different times of development (which can be sensed by certain fetal hairs).
Skin Development and Keratinization
The fetal skin first develops as one layer of cells (from ectoderm) and gradually builds to three layers. Cells of the outer most layer will form part of the vernix (see Lanugo, below) that protects the skin during gestation.
The process of keratinization gives the skin a hard-protective cover, or essentially makes it “waterproof”, “solid”, and “impermeable". The fetal skin is non‐keratinized until weeks 19 to 25, allowing the skin to act as a membrane through which amniotic fluid can pass through.
Prior to complete keratinization, the skin is pinkish-red in color because the blood in the capillaries is visible. It is estimated that newborn skin contains 20 times more blood vessels than it needs to support itself. This excess is likely for the regulation of body temperature. Much of the permanent vasculature of the skin develops in the first few weeks after birth.
After keratinization of the skin, amniotic fluid is no longer absorbed through the body of the fetus, but primarily through swallowing after this time.
The keratinization process may also affect scarring. For example, a fetal incision made early in gestation will heal without a scar, whereas one made in late gestation heals with a scar.
If tissue injury occurs, the fetal skin has the ability to regenerate a collagen matrix that is identical to that of the original tissue. It is also possible that amniotic fluid itself may contain factors that appear to minimize scarring. This aspect of fetal skin is currently being researched to better understand wound healing in adults.
Skin Patterns and Prints
Finger, toe, feet, and palm prints appear around 13 weeks of pregnancy and are permanently in place by 21 weeks.
Fingerprints are determined by genetics, and certain chromosomal disorders can affect these patterns. For example, about 50% of infants with Down Syndrome have distinctive patterns on their hands and feet.
The entire body and its various patches of skin patterns (i.e. fingerprints, eyelids, palms, soles of the feet, and trunk) are in place by 20 to 24 weeks of pregnancy. Each skin pattern is specific to a particular part of the body. For example, even if skin is transplanted to another part of the body, it will keep its original pattern.
Preterm Skin
The skin of extremely preterm infants is underdeveloped and has poor barrier function because the outermost layer does not fully develop until late in the third trimester. In order to combat the weak barrier of preterm skin, plastic wraps may be used to reduce the risk of hypothermia, and adhesive skin dressings can help protect the skin until it matures on its own.
Hair Development
Hair follicle development begins around 9 to 12 weeks of pregnancy. It is believed that all hair follicles an individual will have are present by five months of gestational age and that at birth, a newborn has around 5 million hair follicles on its body.
Around 14 weeks of pregnancy, hair growth starts on the scalp, moves toward the face (around the eyebrow, nose, upper lip, chin, and forehead area), and then proceeds from head to toe. It grows over the entire fetal body at the same rate, so the hairs are the same length.
By 21 weeks, hair can be recognized in various stages of development on all areas of the fetus’ body.
Eyebrows and head hair are visible by 22 weeks, and head hair is well developed by 28 weeks of pregnancy.
NOTE: Around 2 to 3 months of age, the first hairs on an infant may shed over an area on the back of the head; this is often mistakenly thought to be due to head rubbing (especially as babies sleep and spend time on their backs), but is actually a natural process in the hair cycle.
Lanugo
Lanugo (fine, soft, and colorless hair) is initially formed around 14 weeks of pregnancy and helps bind vernix to the skin. Vernix is the sticky white covering present on some newborns that protects fetal skin from anything damaging in amniotic fluid (such as urine). Without lanugo, the vernix would not stick, and the skin would be unprotected. Vernix is also theorized to help facilitate delivery.
The process of lanugo development is generally completed on the fetus’ back at 23 weeks, and on the abdomen by 26 weeks.
Interestingly, the swaying of lanugo hairs during fetal movements in amniotic fluid activates certain triggers of the central nervous system, creating an oxytocin effect that can stimulate growth; this growth then slows down at the end of pregnancy when these hairs are shed.
Lanugo hair is replaced in late pregnancy (33 to 36 weeks) or early newborn life by fine, thin hairs all over the body, but thicker hair on the scalp.
The lanugo that is shed during pregnancy becomes incorporated into amniotic fluid and then becomes part of what will make up meconium, or the baby's first bowel movement after delivery.
Lanugo is still present in up to 30% of newborns, most often occuring with preterm infants.
Nails
Fingernails and toenails begin to develop at the tips of the digits around 12 weeks; fingernail development is ahead of toenail development by almost 4 weeks.
The fingernails reach the fingertips by approximately 34 weeks of pregnancy (and the toenails at 38 weeks). There is no difference in nail length between male and female fetuses. However, nails that have not reached the tips of the fingers or toes by birth generally indicates prematurity.
Overall, newborn nails tend to be very thin, soft, spoon-shaped, and break very easily.
Teeth
Information on the development of teeth is located here (Face).
Resources
Fetal Skin Development (UNSW Australia/Embryology; Dr. Mark Hill)