The concentrations in milk of the major divalent ions are species specific. The calcium level increases markedly during the first few days postpartum but then decreases gradually over the course of the lactation. Citrate and phosphate concentration rise in parallel with the sharp increase in milk volume between 2 to 4 days postpartum. The calcium-to-phosphorous ratio is lower in cow's milk ( 1:4 ) that in human milk (2:2). Lactation also affects the mother 's calcium movement. Calcium uptake in maternal duodenum is enhanced during lactation. After weaning, women who have lactated show significantly more bone in the lumbar spine than women who have not lactated