Melanocytic nevi in pregnancy: histologic features and Ki-67 proliferation index.

Background: Changes in the clinical appearance of benign dermal nevi during pregnancy may be concerning for malignant transformation. Because the hormonal milieu of pregnancy has not proven to alter their banal behavior, histologic characterization is needed to prevent over-diagnosis and unnecessary treatment.
Methods: Dermal nevi excised from pregnant women (n = 16) were compared with nevi from location- and aged-matched control patients (n = 15). Histologic features and Ki-67 proliferation index were evaluated.
Results: Nevi in pregnancy were more likely to have dermal mitotic figures (62.5% vs. 13.3%, p = 0.028) and higher mitotic rates (1.44 vs. 0.20 mitoses/mm(2), p = 0.0027) than control nevi. A distinctive histologic entity, termed superficial micronodules of pregnancy (SMOPs), was observed more frequently in the nevi of pregnancy (81.3% vs. 26.7%, p = 0.040), and showed consistent immunoreactivity for HMB45. There was a trend toward higher Ki-67 proliferation index in the nevi of pregnancy (3.0% vs. 1.0%, p = 0.073). Prominent multinucleated melanocytes were seen only in controls. There was no significant difference in pigmentation or irritation changes between groups.
Conclusions: Dermal nevi removed during pregnancy show characteristic histologic features including increased dermal mitoses, superficial micronodules of pregnancy (SMOPs), and trend toward increased Ki-67 proliferation index.