Besides glycolic acid peels (which I would try first) over the counter products containing kojic acid can help with melasma due to the bleaching effects of the kojic acid. Dermatogists often prescribe either Retin-A or Tri-Luma. You MUST wear sunscreen all the time with these products because if you don't the melasma will come back and sometimes comes back even darker. The links below will give you some info on Retin-A & Tri-Luma. They both can have side effects but the Tri-Luma can have serious side effects that can be severe in a small percentage of people particularly those who suffer from asthma.
tretinoin (Retin-A, Atralin, Renova, Avita, Altinac) Drug Information on MedicineNet.com
What Is Tri-luma Cream? | Topical Therapy For Melasma
I used Tri-Luma for a short time on a sunspot but I would not recommend it due to the possible side effects. I experienced redness and a slight burning so I stopped it and switched to Retin-A. It works slowly and you must be consistent applying it but it is far safer than Tri-Luma and much better tolerated by the majority of people. This info came from the two derms I consulted for this.
If I had the "mask of pregnancy" I would wear makeup over it and see if it fades on its own after a year.