Deciduous teeth do not show fluorosis because:

Option 1: Placenta acts as a barrier. Some authors have said that the placenta acts as a complete barrier to fluoride, others have said that it is only partial. Some have said that the placenta only acts as a barrier, when there is a sudden increase in the maternal plasma fluoride level. Option 2: Fluoride is taken up by the fast-growing skeletal tissue. Fluoride’s actions on bone cells predominate as anabolic effects both in vitro and in vivo. More recently, fluoride has been shown to induce osteoclastogenesis. Option 3: The calcification time of deciduous teeth is less.