In the pre-expulsive and expulsive phases of labor, oxytocin and several other osteoclastogenic mediators, such as prostaglandins and IL-6, are secreted in high concentrations. This study was undertaken to assess whether the peripheral blood obtained from healthy women after vaginal delivery contains a larger pool of osteoclast precursors compared with age- and gender-matched controls. Our results clearly show that the number and size of osteoclasts generated in vitro from osteoclast precursors isolated from women after delivery are significantly larger than those from controls. This finding can account for the decrease in bone mass that is often observed during the breastfeeding period and the concomitant release of high quantities of calcium in the milk. Further investigations are required to establish whether analysis of blood osteoclast precursors can be predictive of changes in bone remodeling in this setting. (c) 2005 Elsevier Inc. All rights reserved.