Where did the lanugo go?
Babies are born with all the hair follicles they will need for their lifetime, about five million. Each of those follicles produces, and then sheds, one tiny strand of hair (called "lanugo") in the uterus before the baby is born.
Baby hair and heartburn
An old wives' tale predicts that if a mother has heartburn during pregnancy, the baby will be born with a full head of hair. In 2006, a group of scientists decided to put this folklore to the test. To their surprise, they found that the tale might be true. Mothers who experienced moderate to severe heartburn were more likely to have babies with thicker hair at birth, and vice versa.
Baby hair growth in the womb
Hair begins to grow around week 10 of pregnancy, and by week 20 the scalp is covered with hair. This first round of hair is called "lanugo" and it is shed in the uterus around 24-28 weeks of pregnancy. This means that any hair a baby is born with was likely grown during the last trimester of pregnancy.