The Linea Nigra is the dark line known as hyperpigmentation, that appears over the belly during pregnancy. It can run from the pubic bone to the belly button or all the way up to the breast bone.
While the exact cause is unknown there are a number of areas where the skin darkens. Not just Linea Nigra but the nipples and areola; the vulva, freckles and scars. It is believed to be associated with the increasing pregnancy hormones & melanin (there hormone that gives skin it’s colour) which rises as well. Those with darker skin tend to have more pronounced pigmentation of the linea nigra.
There is no recognised treatment for linea nigra, as it is a normal pregnancy change with no adverse outcomes. Generally the line lightens again after pregnancy. There is questionable linkage to folic acid consumption that may reduce the darkness of the colour but this has not been substantiated fully. And if you’re concerned about the darkness, then the general advice is to avoid sun exposure to the belly as it may darken.