An increase in circulating, 1,25-dihydroxyvitamin D level and net intestinal calcium absorption have been previously demonstrated in pregnant women and have been widely regarded as compensatory mechanisms whereby fetal mineral demands are satisfied. The alternate possibility, that these adjustments might anticipate such demands, has not previously been considered. To examine the effects of pregnancy on the intestinal absorption and renal excretion of calcium, oral calcium tolerance tests were performed and urinary calcium excretion was measured in 16 healthy women receiving a moderate calcium intake during and after pregnancy. Circulating 1,25-dihydroxyvitamin D levels and indexes of parathyroid function were also measured. As expected, 1,25-dihydroxyvitamin D levels were significantly (p less than 0.05) elevated throughout pregnancy (94 +/- 11, 118 +/- 9, and 117 +/- 11 pg/ml in the first, second, and third trimesters, respectively, versus 51 +/- 5 pg/ml after delivery). Twenty-four-hour calcium excretion also increased sharply (247 +/- 54, 316 +/- 42, 300 +/- 61 mg versus 91 +/- 18 mg), often to the point of hypercalciuria. Calcium tolerance test results included significant increases in the calciuric and calcemic responses during each trimester, whereas fasting calcium excretion and parathyroid function remained normal. These findings portray normal pregnancy as a state of physiologic absorptive hypercalciuria and call into question the widespread practice of supplementing calcium intake in otherwise well-nourished women during pregnancy.