Thanks for the query.
Although the most common causes for such type of dark spots is exposure to UV rays of sun,pregnancy
But since these spots are getting more dark in spite of protection from sun,you must get yourself examined for conditions like addison's disease or pituitary tumor or hemochromatosis
etc,to be on safe side.
As far as treatment is concerned ,beside using sunscreens,you can go for skin lightening creams containing hydroquinone
or chemical peels
or even laser treatments.
Consult a dermatologist
and get yourself examined properly.
Once a proper cause is found,a proper treatment can be instituted.
I hope it helps however you may revert to me for any further query.
Best of luck