As only sparse data are available, we aimed to investigate whether needs for activated vitamin D and calcium supplements change in women with hypoparathyroidism during pregnancy and lactation and risk of pregnancy related complications.Retrospective review of medical records.12 Danish and Canadian patients with chronic hypoparathyroidism who completed 17 pregnancies.Data were extracted on plasma levels of ionized calcium (P-Ca2+ ) and doses of active vitamin D and calcium supplements during pregnancy (N=14) and breastfeeding (N=10). Data on pregnancy complications were available from all 17 pregnancies.Although average doses of active vitamin D (p=0.91) and calcium supplements (p=0.43) did not change during pregnancies, a more than 20% increase or decrease in dose of active vitamin D was needed in more than half of the pregnancies in order to maintain normocalcemia. Five women (36%) developed hypercalcemia by the end of pregnancy or start of lactation. Median levels of P-Ca2+ increased from 1.20 mmol/L in third trimester to 1.32 mmol/L in the post-partum period (p<0.03). Accordingly, the average dose of active vitamin D was significantly reduced (p=0.01) during lactation compared to 3rd trimester. One woman developed severe pre-eclampsia (6%). Further four pregnancies (24%) were complicated by polyhydramnios, dystocia and/or perinatal hypoxia. Ten pregnancies required cesarean delivery (59%) with four (24%) being performed as an emergency.In chronic hypoparathyroidism, close medical monitoring of the mother with frequent adjustments in the dose of calcium and active vitamin D is required during pregnancy and lactation in order to maintain normocalcemia. Patients should be offered close obstetric care to handle potential perinatal complications. We recommend evaluating the neonate immediately after birth and notifying the pediatrician of the risks of hypocalcemia as well as hypercalcemia in the neonate.