Summary The hair diameter major-axis has been shown to decrease normally toward the scalp in individual fibres. In this report sharp increases, rather than decreases, of hair-shaft major-axis diameters during pregnancy are described. Mean major-axis hair diameters of 15–20 fibres from each of 12 pregnant and 13 nonpregnant women were measured from the scalp distally at regular 6-mm intervals. Sample diameters were normalized relative to putative times of conception or a distance from the scalp equivalent to full term pregnancy, for the pregnant and nonpregnant women, respectively, and averaged. Major-axis diameter increases toward the scalp in hair of pregnant women were found to start at the beginning of pregnancy with a slope measuring 0.58%/cm, which is highly significant (r = 0.99, P < 0.0001). Hair diameters of the nonpregnant women, on the other hand, decreased toward the scalp during analogous times to the pregnancy, with a slope measuring −0.66%/cm, also highly significant (r = −0.97, P < 0.0001), and in good agreement with previously published data. To our knowledge this is the first description of hair diameter increases during a normal physiological process.