We used the calcitonin/calcitonin gene-related peptide a gene knockout model (Ct/Cgrp null) to determine if calcitonin and CGRP are required for normal fetal mineral homeostasis and placental calcium transfer. Heterozygous (Ct/Cgrp^+/-) and Ct/Cgrp null females were mated to Ct/Cgrp^+/- males. One or two days before term, blood was collected from mothers and fetuses and analyzed for ionized Ca, Mg, P, parathyroid hormone (PTH), and calcitonin. Amniotic fluid was collected for Ca, Mg and P. To quantify skeletal mineral content, fetuses were reduced to ash, dissolved in nitric acid, and analyzed by atomic absorption spectroscopy for total Ca and Mg. Placental transfer of ⁴⁵Ca at 5 minutes was assessed. Ct/Cgrp null mothers had significantly fewer viable fetuses in utero as compared to Ct/Cgrp^+/- and wt mothers. Fetal serum Ca, P, and PTH did not differ by genotype, but serum Mg was significantly reduced in null fetuses. Placental transfer of ⁴⁵Ca at 5 min was normal. The calcium content of the fetal skeleton was normal; however, total Mg content was reduced in Ct/Cgrp null skeletons obtained from Ct/Cgrp null mothers. In summary, maternal absence of calcitonin and CGRP reduced the number of viable fetuses. Fetal absence of calcitonin and CGRP selectively reduced serum and skeletal magnesium content, but did not alter ionized calcium, placental calcium transfer, and skeletal calcium content. These findings indicate that calcitonin and CGRP are not needed for normal fetal calcium metabolism, but may regulate aspects of fetal Mg metabolism.