Re: 2 Seperate Questions...
Birth marks and such are caused by increased melatonin (i think that is how you spell it) in your skin in a certain place. What I have always considered a beauty mark is a mole, and I don't know what causes them.
If you mean why do they get worse in pregnancy it's because the hormones that your body make extra of can change your skin tone, I can't remeber what it's called, but it's the same thing that causes that line some women get down there bellys and the "mask of pregnancy" the red stuff on your face.
My breasts leaked for a while at like 11 weeks, then quit. Then they started again at about 30 weeks, and still do, but only one of them!