K.N.
Hmm, interesting... I found this on the Internet. Hope it helps:
My two and a half year old daughter has a linea nigra. To the best of my knowledge, she is incredibly healthy. I recently read that a linea nigra in non-pregnant females may be the result of diabetes, insulin resistance or a sex hormone imbalance due to drugs or disease. Do you know anything about this?
Answer:
Linea nigra, a brown stripe running from the belly button to the pelvis, is a fairly common finding in newborn babies (usually African American) and is usually attributed to exposure to maternal estrogens. If this line has been present in your child since birth or soon after, it is likely just persistent since this time. If it has appeared recently, I would discuss it with your pediatrician and see whether your child has other signs of increased estrogen (such as a growth spurt or breast development). I've never heard of linea nigra being a presenting sign of early puberty, however. There is also an association of linea nigra (with other hyperpigmentation) in children with adrenal insufficiency. I am not aware of an association with diabetes in young children.