The Linea Nigra is supposed to be there for everyone from the very start and does not just appear all of a sudden. The only difference here is that this light line turns into a darker line during the trimester of your pregnancy. This is resulted due to the pigmentation that takes place and the rising amounts of the hormonal changes that a mother-to-be experiences. In order words, this linea nigra is caused by the increased melanocyte-stimulating hormone made from the placenta which is caused by melasma and darkened nipples.