I have the same problem of darkening skin in my armpits when I was pregnant and it will still be there after giving birth. It is due to hormones during pregnancy. My beautician recommended me to use her oxygen scrub which works well and the darkening disappears soon after. Maybe you can try using body scrub on yr armpits when bathing. If it does not work after a few weeks, you cazn email me and I will let you know the product which I am using that helps to get rid of it.