The object of this study was to determine whether increased circulating levels of parathyroid hormone-related protein (PTH-rp) may explain the increased parathyroid hormone (PTH) bioactivity in pregnancy. In 41 healthy pregnant women (age 19-41 years), PTH-rp and corrected calcium levels were measured and compared with those of nonpregnant control women (n = 18, age 20-39 years). PTH-rp and corrected calcium levels were significantly higher in pregnant women (PTH-rp 21.9 +/- 7.9 pg/ml, P < 0.001; corrected calcium 2.38 +/- 0.07 mmol/liter, P = 0.001) than in nonpregnant women (PTH-rp 10.3 +/- 7.8 pg/ml; corrected calcium 2.30 +/- 0.10 mmol/liter). Our data indicate that circulating PTH-rp levels may significantly increase in pregnancy, suggesting a possible role of this peptide in the modification of calcium homeostasis in pregnant women.