Background: Osteonecrosis is usually associated with trauma, use of corticosteroids, or alcohol abuse. We investigated the rare association of osteonecrosis of the femoral head and pregnancy, and we defined differences between the disorder in pregnant women and that in women of childbearing age who were not pregnant. The results of treatment with a free vascularized fibular graft were evaluated in terms of relief of pain and improvement of the Harris hip score after a minimum of two years of follow-up.Methods: Thirteen women (seventeen hips) had the onset of pain in the hip during pregnancy or within the first four weeks after delivery, and the pain persisted until a diagnosis of osteonecrosis of the femoral head was made on the basis of magnetic resonance imaging. No patient had any other risk factor for this disease. Information was obtained by means of clinical assessment, a review of the records and radiographs, and a telephone survey. Eleven women (fifteen hips) were managed with a free vascularized fibular graft, and nine of them (eleven hips) were evaluated, with regard to relief of pain and the Harris hip score, at a minimum of two years postoperatively.Results: The average age when the pain began was 31.5 years (range, twenty-five to forty-one years). Eleven of the thirteen women were primigravid, and the patients typically first had the pain late in the second trimester or in the third trimester of pregnancy. The women tended to have a small body frame and a relatively large weight gain during the pregnancy. Eight of the thirteen patients had swelling and varicosity of the lower extremities. The diagnosis was delayed an average of 10.3 months, with a range of three to thirty months. A common misdiagnosis was transient osteoporosis of the hip during pregnancy. A correct diagnosis was established for all hips on the basis of the finding of a double-density signal on magnetic resonance imaging or evidence of progression of the disease on plain radiographs. According the system of Marcus et al., the stage at the time of diagnosis ranged from II to V. All women had involvement of the left hip, and four had bilateral involvement.Of the eleven women (fifteen hips) who were managed with a free vascularized fibular graft, nine noted marked or complete relief of the preoperative pain. Two hips in a patient who had progressive pain were treated with a total hip arthroplasty. Two hips (one patient) were lost to follow-up. The nine patients (eleven hips) who were available for follow-up at a minimum of two years had an average improvement in the Harris hip score of 24 points.Conclusions: Occasionally, pain in the hip that begins during pregnancy is caused by osteonecrosis of the femoral head. A high index of suspicion and use of magnetic resonance imaging may lead to an earlier diagnosis and a better prognosis in this population of women. In this study, treatment with a free vascularized fibular graft was a useful option with which to obviate or postpone the need for total hip arthroplasty.