Though significant amounts of bone mineral can be lost during breastfeeding, the loss of bone tends to be transient. Studies have consistently shown significant trends in bone loss during lactation, with full recovery of bone density by six months after weaning. Kalkwarf and Specker reported women who experience an earlier resumption of menses lose less bone during lactation and recover more bone after weaning. Other studies have identified similar trends in bone loss, with full recovery of bone density by six months following the cessation of breastfeeding. The positive effect on bone density has been seen in both pre and postmenopausal women.
Parity appears to have a protective effect against fracture, as well.
The validity of studies that use nulliparous women as controls has been criticized, however. Many have suggested that nulliparous women may not be appropriate studies of parity since they have not demonstrated the ability to conceive and to support fetal growth and development. Such women may have a hormonal environment that impedes conception and negatively impacts bone density. The National Resource Center is supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases with contributions from the National Institute of Child Health and Human Development, National Institute of Dental and Craniofacial Research, National Institute of Environmental Health Sciences, NIH Office of Research on Women's Health, Office of Women's Health, PHS, and the National Institute on Aging. The Resource Center is operated by the National Osteoporosis Foundation, in collaboration with The Paget Foundation and the Osteogenesis Imperfecta Foundation.