googled it:
Linea Nigra:
What is this? Linea nigra is the dark line that runs from your navel to your pubic bone. This is a line that may have always been there, but you may have never noticed it before because it was a light color. During pregnancy this line darkens and is possibly caused by the imbalance in hormones. It usually appears around the fourth or fifth month of pregnancy.
What can I do? There is nothing you can do to prevent this from happening, but after your pregnancy this line will fade.