Carbon (13C/12C) and nitrogen (15N/14N) stable isotope ratios were longitudinally measured in human hair that reflected the period from pre-conception to delivery in 10 pregnant women. There was no significant change in the δ13C results, but all subjects showed a decrease in δ15N values (−0.3 to −1.1‰) during gestation. The mechanisms causing this decrease in hair δ15N have not been fully elucidated. However, since the δ15N values of dietary nitrogen and urea nitrogen are significantly lower compared to maternal tissues, it is hypothesized that the increased utilization of dietary and urea nitrogen for tissue synthesis during pregnancy resulted in a reduction of the steady state diet to a body trophic level effect by approximately 0.5–1‰. An inverse correlation (R2 = 0.67) between hair δ15N and weight gain was also found, suggesting that positive nitrogen balance results in a reduction of δ15N values independent of diet. These results indicate that δ15N measurements have the ability to monitor not only dietary inputs, but also the nitrogen balance of an organism. A potential application of this technique is the detection of fertility patterns in modern and ancient species that have tissues that linearly record stable isotope ratios through time. Copyright © 2004 John Wiley & Sons, Ltd.