PURPOSE:Intra‐abdominal desmoid tumors occur in 12 percent of patients with familial adenomatous polyposis. A minority grow quickly and are lethal, most are relatively inert, and some cause problems by obstructing adjacent organs. Desmoid tumors may be estrogen‐dependent, and estrogen‐blocking drugs are part of the usual treatment of these tumors. This study was performed to examine the effect of pregnancy on the course of patients with familial adenomatous polyposis and intra‐abdominal desmoids.METHODS:All females with familial adenomatous polyposis and an intra‐abdominal desmoid treated or followed up at this institution were eligible. Stable, asymptomatic desmoids were followed up yearly with examination and CT scan. Growing or symptomatic desmoids were followed up at least every six months. Maximum tumor size was grouped as follows: <10 cm, 10 to 20 cm, and >20 cm. A change in tumor size was defined as a change of ±50 percent or more of maximum diameter. Stable tumors showed no change in diameter during the study period; variable growth was defined as a significant change in either direction that was followed by a return to previous dimensions or a stabilization of growth. Rapid growth was a doubling of diameter within three months. Pregnant females were compared with nonpregnant females. Subgroups of females were matched for age at diagnosis of desmoid.RESULTS:Twenty‐two females had never been pregnant, whereas 25 had been pregnant at least once. Eleven pairs were matched for age. There were no differences between groups in the incidence of extracolonic manifestations of familial adenomatous polyposis, family history of desmoids, number or type of surgeries done for familial adenomatous polyposis, length of follow‐up, or time from surgery to desmoid diagnosis. Desmoids in pregnant females had a significantly more benign course: 18 were stable (vs.6 nonpregnant females), 2 had variable growth (vs.10), 1 had rapid growth (vs.5), and 4 disappeared (vs.1). There were also trends to smaller, less symptomatic tumors requiring treatment less often in pregnant females.CONCLUSIONS:Pregnancy seems to ameliorate the course of abdominal desmoid tumors significantly in females with familial adenomatous polyposis. This finding raises questions about the most appropriate hormonal treatment for these tumors. Perhaps progesterone or prolactin therapy should be tried, alone or in combination with estrogen. If further studies confirm these findings, females with a family history of desmoid tumors should not be advised against pregnancy.