I have got pimples on my chest and darkened armpits which i don have before pregnancy. Any idea how to get rid or improve the situation?

It's the pregnancy hormones. The pimples will go away and your underarms will lighten after pregnancy. For now, just maintain good hygiene so the pimples don't get worse. And for the darkening of underarms? Not much you can do to control that but I wouldn't apply anything with strong chemicals that could aggravate it further.