812: Do fibroids change in size during pregnancy?

Uterine fibroids are a common finding in pregnant women. The data on how fibroids change during pregnancy, however, is limited. The purpose of this study is to characterize the effect of pregnancy on uterine fibroids. A retrospective cohort study of pregnant women with fibroids who were identified through our computerized ultrasound database over a period of 6 years. Gestational age, fibroid quantity, location, and volume (individual and collective) were reviewed. Fibroids without serial measurements were excluded. The primary outcome was change in fibroid size across trimesters. Secondary outcomes were the associations of initial fibroid size and the quantity of fibroids on growth. Standard statistics were applied with a p- value of < 0.05 considered significant. In 145 pregnant women there were 229 fibroids analyzed. Fibroid size ranged from 5.23– 5670 cm3 in the first trimester and between 2.89-13,248 cm3 in the third trimester. The median individual fibroid volume was 90.1 cm3 (interquartile range 36.3 – 198 cm3) and 69.3 cm3 (interquartile range 32.4–179.7cm3), in the 1st and 3rd trimesters, respectively. The median fibroid volume grew by 26.6% between the 1st and 2nd trimesters, but decreased by 24.4% between the 2nd and 3rd trimesters, and overall decreased by 15.4% between the first and last ultrasound measurements. Fibroids with an initial volume below the median were more likely to grow than those that measured above the median (57% vs 28%; p< 0.001). Of the 28 fibroids identified in the lower uterine segment during the first trimester, only 6 (23%) remained in the lower uterine segment during the third trimester. Smaller fibroids measured in the first trimester are more likely to grow throughout pregnancy compared to larger fibroids. The majority of large fibroids do not grow during pregnancy. First trimester lower uterine segment fibroids are likely to change position by the end of pregnancy.