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What is Osteoporosis? Osteoporosis is a condition characterized by weak and brittle bones. When a person has osteoporosis, the bones can become so brittle that activities like bending over, lifting a vacuum or coughing can cause a fracture. In most cases, the brittle and weak bones are caused by low levels of calcium, phosphorous and other minerals in your bones. Pregnancy-Associated Osteoporosis Pregnancy-associated osteoporosis was first reported more than forty years ago. At that time researchers had identified four post-pregnancy patients with varying degrees of spinal osteoporosis. In 1955, researchers reported five women who experienced vertebral fractures following delivery. While other cases of pregnancy-associated osteoporosis have since been identified, studies surrounding pregnancy and bone have offered conflicting results, including no changes in bone density, and even increases at specific sites. It appears that if pregnancy-associated osteoporosis does exist, it is probably quite rare. (By 1996, eighty cases had been reported in the literature.) It is also a condition that is difficult to adequately investigate, given the inability to perform maternal radiologic exams. Pregnancy-associated osteoporosis tends to be identified in the postpartum period (56%) or the third trimester (41%). Affected women usually present with back pain, loss of height, and vertebral fractures. Hip pain and fracture of the femur are less common but have been reported. The condition usually appears during the first pregnancy, tends to be temporary and usually does not recur. It is not entirely clear whether this disorder is a consequence of pregnancy or whether it occurs because of pre-existing conditions in the pregnant woman. Studies have shown that only a minority of these patients have risk factors, such as heparin or glucocorticoid exposure. On the other hand, it is possible that genetic factors play a role in the development of pregnancy-associated osteoporosis. In a study by Dunne and associates, mothers of patients with the condition had significantly more fractures than controls. Theoretically, pregnancy-associated osteoporosis is believed to occur because of the stress on maternal calcium stores and an increase in urinary calcium excretion. Yet, the intestinal absorption of calcium is increased during pregnancy--especially in the second and third trimesters. The body also responds to fetal calcium demands by increasing total 1,25-dihydroxyvitamin D levels. These two mechanisms help to satisfy the increased demand for calcium during pregnancy. Other physiologic changes during pregnancy that may actually be protective of bone include the third trimester estrogen surge and increased bone-loading due to weight gain. Clearly, there is much to be learned about bone remodeling during pregnancy and why some women become prone to bone loss and even fracture. Lactation and Bone Loss Studies have shown that the majority of women with pregnancy-associated osteoporosis are breastfeeding at the time of diagnosis. Duration of lactation has ranged from one week to seven months. Bone loss tends to be greatest in skeletal sites with the highest concentration of trabecular bone. Reductions in bone density by three to five percent at the lumbar spine are common. Two physiologic occurrences may be responsible for bone loss during lactation. First, there is an increased calcium demand from maternal bone. This demand varies from woman to woman based on the amount of breast milk produced and upon the duration of lactation. Secondly, because of elevated prolactin levels, women who breastfeed tend to be in a hypoestrogenic state. 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. Parity and Osteoporosis Risk Much
of the research reports an increase in bone density with increasing parity (the
number of pregnancies a woman delivers past 28 weeks gestation.) 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. In an arm of the
Leisure World study, Paganini-Hill and others concluded that women with three
or more children had a 30-40 percent reduction in hip fracture risk when compared
to nulliparous women (those who have never given birth). 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.
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