I understand your concern.  The overall beauty of a woman is generally enhanced during pregnancy, but such problems good looks. Is this a single rash or there many more, is it dark coloured or red and angry looking? Is there associated itching and or pain or there is no other problem except the rash?

Pregnancy causes change in level of hormones with surge in melanocyte (skin pigment) producing hormone. This can cause a rash on the face on one side or both sides, which is known as melasma. This rash does not cause any problem except for anxiety due to its appearance. It resolves spontaneously after child birth. Exposure to sunlight, thyroid hormone imbalance, stress, both physical and mental, can aggravate it.  A genetic susceptibility is also involved. Usually somebody having melasma during her first pregnancy can have it during subsequent pregnancies also.

If there is a localised red, angry looking rash associated with pain and or itching, it can be due to local inflammation over that site. It can be due to a minor injury like a cut or scratch or insect bite or even bacterial infection. Allergy to some products used can also cause a localised rash.  

Cosmetic products containing strong chemicals when used over the face can also cause local irritation and subsequent rash production.

Fungal infections over the face can also manifest as rashes. But these are usually associated with itching.  Rash can also be due to dryness of the skin. Pre existing acne may get worsened during pregnancy.

Increased blood flow that occurs during pregnancy can lead to widening of small blood vessels which may look like blotches or rashes anywhere on the body, including the face.

But there is nothing to worry about. Avoiding exposure to strong sunlight by using scarves and sunglasses and being relaxed physically and mentally can keep melasma to the minimum possible. But there is no cure available for it. Maybe some face foundation can be tried to mask it.

Avoid using face creams and lotions with strong chemicals that can possibly cause allergy. Drinking plenty of water and using a soft moisturising agent over the skin can prevent dryness and resultant itching and rash. In case of worsening of acne, an antibiotic ointment like erythromycin can be used locally over it.

 

Antifungal ointments will help to cure if the rash is a localised fungal infection.

Best way is not to worry about it. After delivery it should improve. But in case the rash persists a dermatologist can be consulted.