What is linea nigra?
Linea nigra is a physiological form of hyperpigmentation commonly seen in the first trimester of pregnancy [1–3]. It is a dark vertical line that runs down the middle of the abdomen and it can be one of the earliest indicators of pregnancy [1,3]. It is also known as the ‘pregnancy line’.
Who gets linea nigra?
Linea nigra occurs in more than 90% of pregnant women, and is often in association with hyperpigmentation of nipples, areola, and genital areas [2,4–8].
Pregnancy-related hyperpigmentation is prominent and more common in women with darker complexions (Fitzpatrick skin types 4–6) compared to fair-skinned women (Fitzpatrick skin type 1 or 2) [2,9].
Linea nigra can also affect men and children.
A study involving 1550 Nigerian patients reported:
- The incidence of linea nigra was 92.0% in pregnant women and 16.0% in non-pregnant women .
- Linea nigra affected 80% of men older than 50 years of age who had benign prostatic hyperplasia or prostate cancer.
- There was an increasing incidence with age in children, reaching a peak of 45% between 11 and 15 years of age .
What causes linea nigra?
Linea nigra and other forms of pregnancy-related hyperpigmentation are due to placental hormones, metabolic factors and immunological factors .
- Increased production of melanin is due to the effects of oestrogen, augmented by progesterone [1–3,6,11].
- Oestrogen activates intracellular oestrogen α- and β-receptors located in in the skin, stimulating melanocytes to increase melanin output .
- Melanin is deposited in the epidermis and taken up by dermal macrophages .
- The placenta also produces bioactive sphingolipids, which upregulate melanogenic enzymes such as tyrosinase and tyrosinase-related proteins 1 and 2 .
It is no longer thought that α– and β–melanocyte-stimulating hormone from the pituitary gland causes hyperpigmentation during pregnancy .
What are the clinical features of linea nigra?
Linea nigra is a linear band of macular hyperpigmentation that extends vertically down the midline of the abdomen [13,14].
- Linea nigra usually extends from the symphysis pubis (the cartilaginous joint between the pubic bones) to the umbilicus (the belly button), but it can extend further up to the xiphoid process (the lowest point on the sternum) [4,6,9,11].
- Hyperpigmentation can range from a slight shade of brown to a grayish black [10, 12]. It may darken following sun exposure.
- The width of linea nigra is usually around 1 cm; it is consistent in width along its entire length .
- Linea nigra can increase in width and intensity throughout pregnancy.
- It is often accompanied by the 'ligamentum teres sign', when the umbilicus deviates to the right .
How is linea nigra diagnosed?
Like other cutaneous physiological changes of pregnancy, linea nigra is diagnosed clinically . No specific tests are necessary.
What is the differential diagnosis for linea nigra?
Occasionally, other forms of localised hyperpigmentation may be considered in the differential diagnosis of linea nigra .
Other pigmented conditions that may occur on the abdomen include:
- Café au lait macules 
- Postinflammatory hyperpigmentation 
- Flagellate hyperpigmentation associated with exposure to bleomycin, peplomycin, and docetaxel; dermatomyositis; adult-onset Still disease; and Shiitake mushroom dermatitis 
- Pigmented skin lesions, such as melanocytic naevi (moles), seborrhoeic keratoses, and lentigines 
- Previous or current superficial skin infections, such as pityriasis versicolor and erythrasma.
What is the treatment for linea nigra?
There is no specific medical treatment for linea nigra . Women can be reassured that linea nigra has no adverse effect on pregnancy outcomes, and medical treatment is not required . Affected women should avoid sun exposure to the abdomen, as this might cause the line to become darker.
It has been suggested that folic acid reduces the formation of linea nigra. Folic acid can be found in foods such as leafy green vegetables, oranges and whole wheat bread . Bleaching has been used in some cases, with unsatisfactory results [13,19,20].
What is the outcome for linea nigra?
Linea nigra usually gradually fades over time after delivery, but some women may have persistent hyperpigmentation [1–3]. It can reappear with subsequent pregnancies .