['Early childbearing may limit skeletal consolidation and increase calcium demands in adolescents.', 'The purpose of this study was to characterize calcium absorption in pregnant and lactating adolescents.', 'Fractional calcium absorption was evaluated in 23 adolescents (mean +/- SD age: 16.5 +/- 1.4 y) during the third trimester of pregnancy (34.7 +/- 1.0 wk gestation) and again in 15 of these adolescents 31 +/- 8 d after delivery. Eight adolescents were breastfeeding their infants during the follow-up study. Fractional calcium absorption was determined by using oral ((46)Ca or (44)Ca) and intravenous ((42)Ca) stable calcium isotopes. Total-body and lumbar spine bone mineral density were measured in adolescents during the postpartum period by using dual-energy X-ray absorptiometry.', 'Fractional calcium absorption was significantly greater during pregnancy than at 3-4 wk postpartum [0.526 +/- 0.152 (n = 23) compared with 0.297 +/- 0.108 (n = 15); P < 0.0001]. Lumbar spine z scores measured 19-44 d after delivery (n = 15) were significantly associated with calcium intake during pregnancy (y = -3.53 + 0.107x; R(2) = 0.355, P < 0.02) and were inversely related to fractional calcium absorption during pregnancy (y = 3.489 - 6.66x; R(2) = 0.52, P = 0.002). A total of 33% (5/15) of adolescents had lumbar spine z scores that met the definition of osteopenia (n = 3) or osteoporosis (n = 2) in the early postpartum period.'] Is calcium absorption significantly higher in adolescents during pregnancy than in the early postpartum period?