Uneven skin color on face

Currently 21 weeks pregnant (5th pregnancy) I’ve just noticed that my forehead has a distinct line that divides my upper forehead and bottom. Mas dark yung area on the upper part. Not just my forehead but the outline of my face. While the middle part is much lighter. Mejo itchy din sa mga dark parts pero wala nman red spots or itch marks. Just feels really itchy. Ano kaya ito mga Mamsh? 1st time ko kasi maexperience ito, compared to my past 4 pregnancies.