Melasma has more than one possible trigger. This article walks through two that a published source names, then shows how to turn them into a sharper question for a clinic.
Sunlight begins the list
www.aad.org lists the possible melasma triggers, and the first entry is sunlight: "When sunlight hits our skin, it triggers the body to produce more melanin."
For a plain overview of the condition itself, see melasma (facial pigmentation).
Hormones during pregnancy
The same list names pregnancy: "An increase in the hormones estrogen and progesterone, which occurs during pregnancy, is thought to trigger melasma."
Not every dark patch is the same condition
Dark patches are not all one condition. For the wider topic of darker areas of skin, see this overview of hyperpigmentation.
Marks that follow irritation have their own overview as well, listed under postinflammatory hyperpigmentation.
The question that tightens a quote
Ask the clinic what it believes is driving your patches. Then ask how the quote changes if sunlight or hormones are involved. A quote that cannot answer that question is closer to a ballpark than a firm number. For readers in Chelsea, Michigan, bring both questions to a first consultation.