Objectives: To assess cobalamin (vitamin B₁₂) status during normal pregnancy and postpartum in a longitudinal setting. Methods: This study was performed in 1995-1996. It comprised 406 healthy, pregnant Danish Caucasian women, living in Copenhagen County. Cobalamin status, i.e. plasma (P-) cobalamin, P-methylmalonic acid and P-homocysteine was measured at 18, 32 and 39 wk gestation and 8 wk postpartum during lactation. Results: P-cobalamin showed a gradual, significant decline during pregnancy (P < 0.0001) followed by a significant increase postpartum (P < 0.0001); at 18, 32, 39 wk gestation and 8 wk postpartum median values were 225, 172, 161 and 319 pmol/L, respectively. P-methylmalonic displayed a gradual, significant increase during pregnancy as well as postpartum (P < 0.001) with median values of 0.11, 0.13, 0.14, and 0.16 μmol/L, respectively. P-homocysteine demonstrated a significant increase during pregnancy and postpartum (P < 0.001). The frequency of P-cobalamin values <150 pmol/L increased during pregnancy from 15% at 18 wk to 43% at 39 wk gestation and subsequently declined to 3% postpartum. Conclusion: Low cobalamin status may occur among pregnant women, especially in late pregnancy. The recommendations for periconceptional vitamin B₁₂ supplementation should be reconsidered.