I have read it can be very common during pregnancy. It is called chloasma. It's caused from the same hormones that produces that lovely dark line down some of our abdomens, called the linea alba. I got this with DD, so far it hasn't appeared this time around but I didn't get it til around 30weeks last time. It also causes hyperpigmentation, darkending of our areolas . It should go away after pregnancy though, but sometimes very slowly. And Fletch is right, the sun can make them darker.
Got to love all these pregnancy hormones.