The research, published in the May issue of the Journal of Bone and Mineral Research, found women who give birth to at least one child dramatically reduce their risk of hip fracture later in life, compared with women who remain childless.
What's more, the greater the number of babies you have, the greater your protection.
"We don't expect this finding to alter a woman's thinking about her childbearing options, but we do hope it will send an important message to women who do not plan to have children to take steps beginning in their childbearing years to protect their bones later in life," says Dr. Teresa Hillier, an endocrinologist and principal investigator for the study at the Kaiser Permanente Center for Health Research in Oregon.
The new study also found a bone mineral density test (BMD), an important measure of bone strength, did not predict the fracture risk in the childless group.
"A bone mineral density test is a very important indicator of bone health, but clearly, it does not tell the whole story, particularly in women who remain childless," Hillier says.
Although the researchers can't say for sure why pregnancy has this protective effect, Hillier theorizes it may slightly alter the shape of the bones, as well as strengthen some support muscles in the hip, ultimately reducing the risk of hip fracture.
Endocrinologist and osteoporosis specialist Dr. Loren Wissner-Greene calls the study intriguing, and sees several more possibilities for the findings.
"Even though the bone density is the same in both groups of women, it's possible that pregnancy may alter the micro architecture of the bone -- affecting the inner structure in a way that makes it stronger in certain areas of the body," says Wissner-Greene, a professor of endocrinology at New York University Medical Center.
In addition, she says, because weight-bearing exercises have been known to increase bone strength, it's possible the extra weight of pregnancy may help build stronger bones, particularly in the pelvic region.
The new research involved nearly 10,000 postmenopausal women aged 65 and older who were enrolled in the multi-center, U.S. government-funded Study of Osteoporotic Fractures -- the largest long-term observation study of its kind.
The women were divided into two groups -- those who had given birth to one or more child, and those who remained childless. The researchers measured bone density at the wrist, hip and spine, and questioned the women on a variety of health and lifestyle factors. These included their age at the time of their first and last menstrual periods, height and non-pregnant weight at age 25, change in weight since age 25, and calcium intake from ages 12 to 18 and ages 18 to 50.
In addition, the researchers also noted the women's level of physical activity before and after age 25, family history of fractures, whether or not they smoked, their personal exercise history, and the number of hours they spent each day sitting or lying down -- all factors that could influence bone health. The researchers also documented any personal history of hyperthyroidism, diabetes, hysterectomy or oophorectomy (ovary removal), as well as all regular use of medications, including hormone replacement therapy.
History of bone fracture before the start of the study was documented, while regular follow-ups during the study period provided continuing updates on fracture risks.
After adjustments were made for all lifestyle factors, the researchers found women who had one or more babies were 44 percent less likely to experience a hip fracture than women who remained childless. Interestingly, the risk of spine and wrist fractures remained the same in both groups of women, and all the women had similar levels of bone density.
To further tease out the effects of childbearing, the researchers then divided the group of mothers according to the number of children they had had. The result: The greater the number of children, the lower the risk of hip fracture.
"The important message in this finding is that women need to pay attention to bone health beginning in their childbearing years, and both women and their doctors need to take a woman's childbearing history into account when planning preventative care programs," Hillier says.