A nursing baby receives about 200 to 300 milligrams of calcium each day through breast milk. Some comes from the mother’s diet. Much of it is supplied by a reserve hidden in plain sight: her skeleton.

Breastfeeding changes the mother’s hormones in ways that temporarily allow calcium to be released from her bones and carried through the bloodstream to her milk. Studies of nursing women have found declines in bone density of approximately 3 to 10 percent, depending on the part of the skeleton measured and the length of breastfeeding.

This usually represents a temporary withdrawal rather than permanent damage. After the baby is weaned, bone rebuilding accelerates, and most women recover the lost bone within the following 6 to 12 months. Breastfeeding is not generally associated with a greater risk of osteoporosis later in life.

In 2024, researchers identified a possible part of this rebuilding system. In nursing mice, a small group of brain cells began producing a protein called CCN3, which stimulated new bone growth while the mothers were supplying calcium through their milk. When the researchers reduced CCN3 production, the mothers lost substantially more bone.

The findings were not limited entirely to mice. In laboratory experiments, CCN3 also stimulated human skeletal stem cells, encouraging the activity needed to produce bone and cartilage. Researchers have not yet established whether nursing women experience the same temporary surge of CCN3 seen in mice.

For a time, a mother's skeleton helps build another, then quietly restores what it provided.