When you are pregnant why is there a line that across your stomach like band?

The line that appears across a woman's stomach during pregnancy is called the linea nigra. It is a dark, vertical line that runs from the navel to the pubic bone. The linea nigra is caused by the increased production of melanin, which is the pigment that gives skin its color. This increase in melanin is caused by the hormones that are produced during pregnancy. The linea nigra usually becomes visible in the second trimester and it fades after childbirth.