The line on your pregnant belly is called the linea negra. This line is caused from hormones in your body. An old wives tale includes that this only happens to the mother of boys, but I'd personally have to disagree, being as I have five daughters and still had this line every time.
Typically it runs from your pubic bone to the top of your fundus, externally where your belly stops growing. It may be darker or lighter than others but is usually always present. It will fade after you give birth.