Linea nigra is thought to be a consequence of increased production of melanin secondary to increased circulating levels of estrogen, progesterone, and melanocyte stimulating hormone (MSH) in pregnancy. Upregulation by human placenta lipids of tyrosinase has also been hypothesized to play a role. In 90% of pregnant individuals, hyperpigmentation of some type is seen, including darkening of the areolae, ephelides, scars, or melasma. Women with lighter skin phototypes develop hyperpigmentation less often than women with darker pigmentation. Linea nigra tends to appear around the second or third trimester. After birth, the line fades over time, but it may never disappear entirely.
In a population-based study of Nigerians, linea nigra was found to be present in both male and female patients. For males, incidence peaked at 40% in the 11- to 15-year-old age group, whereas in females, incidence was highest in the 16- to 30-year-old group, in which 85% were affected. Pregnant individuals were 60 times more likely than nonpregnant individuals to have linea nigra. In males, an increased incidence has been noted in patients with prostate cancer and benign prostatic hypertrophy. Its use as a screening tool for prostate cancer has been suggested, but it was not found to be a sensitive marker in one study.
Related topic: linea nigra in newborn
L81.8 – Other specified disorders of pigmentation
O26.90 – Pregnancy related conditions, unspecified, unspecified trimester
90751002 – Linea nigra
Differential Diagnosis & Pitfalls
- Café au lait macule – Generally not linear and vertically oriented.
- Postinflammatory hyperpigmentation – Patient would have a history of prior inflammation.
- Flagellate hyperpigmentation – Patient would have history of bleomycin exposure (drug-induced flagellate pigmentation) or shiitake mushroom ingestion (flagellate mushroom dermatitis).
- Melasma – Much more commonly presents on the face and upper body.