Linea negra is an unexplained, but totally benign, feature of pregnancy that is not fully understood.
It is a brown line that extends along the pregnant woman's abdomen, from the belly button to the pubic bone. It normally develops during the second trimester and can range from light to very dark in colour.
Not all pregnant women develop linea negra, but the majority will.
While its purpose is not understood, it is known to be caused by hormones that produce excess melanin during pregnancy. Melanin provides the skin with colour and, in the same way that linea negra is caused, a woman's nipples, moles and freckles may also become darker as a result of the excess melanin.
Dark-skinned women tend to develop a darker linea negra, although fair skinned women can still develop a very dark line. Some babies may even be born with their own linea negra, a phenomenon which is not understood either.
Linea negra usually fades after birth, but can take weeks, months or even years. It can also be permanent.
There is some research that suggests a link between linea negra and a pregnant woman's folic acid intake, and her insulin levels. However, neither of these theories are clinically proven.
Linea negra also features in old wives' tales – if it stops at the belly button, the baby will be a girl and if it continues up to the sternum, the baby will be a boy. This is of course more likely to be pure coincidence!