It's called the linea nigra and it has always been there actually. It's just darker now that you're pregnant. http://sg.theasianparent.com/predict-your-babys-gender-with-old-wives-tales/
It is actually known as linea nigra. After your baby is born, the line should gradually fade all on its own.
It is normal in pregnant women and will gradually disappear after delivery.
I noticed that not only I have it, my baby girl has it too.
thanks