Understanding pregnancy's pigmentary puzzle: The role of demarcation lines

Summary: Pigmentary Demarcation Lines (PDL) are natural, distinct borders between areas of differing skin pigmentation that may become more visible during pregnancy due to hormonal changes affecting melanin distribution. Recent classification of PDL includes eight types (A-H) and is commonly linked to inheritance and cutaneous mosaicism. Distinguishing PDLs from pregnancy-related pigmentation (e.g., melasma, linea nigra) is crucial for accurate diagnosis and avoiding unnecessary invasive tests, especially in patients with skin of color, where cosmetic concerns are more pronounced. We present a case involving a dermatologist who experienced pigmentary changes, likely attributable to PDL. A 41-year-old primigravida of South Asian heritage developed asymptomatic well-demarcated pigmentation on the posterior aspect of lower limbs bilaterally during her second trimester. These symmetrical linear changes had lighter skin medially, transitioning to darker pigmentation laterally, with no changes on the anterior aspect. The obstetric team initially suspected this was an underlying deep vein thrombosis (DVT). However, the patient, a dermatologist by profession, was certain of the clinical diagnosis of PDL and therefore further investigations were not pursued. Where the pigmentation gradually faded over two years, it is likely that these demarcated lines are consistent with type B PDL, which often develops in the third trimester of pregnancy and regresses spontaneously post-delivery. This case report emphasizes the importance of recognizing PDL as a key differential for pigmentary changes in pregnancy, characterized by abrupt, well-demarcated lines, typically on the lower limbs. Prompt recognition of this self-limiting condition prevents unnecessary tests and treatments. Counselling patients on the benign course of these changes helps ease anxiety and address psychosocial and cosmetic concerns, especially in skin of color patients. Mythili Ramamurthy Srinivasan¹, Farishta Khan², Zhenghao Wang², Anila Kapadia² 1. General Internal Medicine, George Eliot Hospital NHS Trust, Nuneaton, England, United Kingdom. 2. Dermatology, Norfolk and Norwich University Hospitals NHS Foundation Trust, Norwich, England, United Kingdom. Pigmentation, Melanoma, and Melanoma Immune Surveillance