Yep, every woman has one though some are darker than others. We have it even before pregnancy but don't see it until we are pregnant because of the same hormones that darken our nipples.
This line is called the linea negra. Its is seen during pregnancy and is normal. As Sun Worshiper said it will disappear after the baby is born.
It's normal in pregnancy no indication of boy or girl or anything else. Some women get it an it goes away after delivery though can take up to a year after delivery to completely disappear.