The incidence of lower mid-trunk hyperpigmentation (linea nigra) is affected by sex hormone levels.

The incidence of linea nigra was studied in 1,550 Nigerians of both sexes and of different age groups and among pregnant women and men with benign and malignant prostatic diseases over a nine-month period. From the study, it would appear that linea nigra increases in incidence from the age group 0-15 years (31.4%) to the age group 16-30 years (47.3%) before starting to fall in clinically normal individuals >30 years to 19.2%. For age groups 0-5 years, 6-10 years and 16-30 years, females more often than males have a linea nigra. For age group 11-15, males are equally as likely as females to have a linea nigra. The results suggest that women over 30 are more likely to have a linea nigra than men, but there are too few patients to make a definite statement, given the number of statistical tests performed. Pregnant women far more often have a linea nigra than nonpregnant women of the same age. The findings suggest that the likelihood of having a linea nigra depends on the level of sex hormones. This means that changes in the levels of hormones, either due to disease or drugs, may be reflected in changes in the incidence of a linea nigra. If this finding is confirmed, the linea nigra may serve as a convenient, noninvasive, free marker of alterations in sex hormones.