Linea nigra (Latin for "black line") refers to the dark vertical line that appears on the abdomen during pregnancy. The brown-colored streak measures about a centimeter wide and runs down the midline of the abdomen, from the rib cage to the pubis.
This line is caused by pregnancy hormones, which increase the production of the pigment melanin. It usually appears around the second trimester and may or may not appear within the first couple months after birth.
An old wives' tale says that if the linea nigra runs to the navel, you're going to have a girl; but if it runs all the way up to your rib cage, you're going to have a boy.