Although the condition is primarily hormonally-driven, exposure to the sun can aggravate the issue, so hats and protective clothing should be worn, and a sunscreen with an SPF of at least 30 is recommended.
Usually completely benign in nature, the hyperpigmentation seen during pregnancy frequently disappears completely, or significantly reduces, after delivery.
Massinde, A, et al. “Extensive Hyperpigmentation during Pregnancy: a Case Report.” Journal of Medical Case Reports, vol. 5, 19 Sept. 2011, p. 464., doi:10.1186/1752-1947-5-464.