Background: Uterine leiomyomas are the leading cause of hysterectomy in the United States, accounting for over 200,000 procedures each year. Most epidemiologic studies of uterine leiomyoma show that parity has a protective association with leiomyoma, but the mechanism is not known. Both epidemiologic data and data from an animal model indicate that the protective association is not an artifact resulting from reduced fertility among women with fibroids. We hypothesize that the process of uterine regression following delivery results in loss of small fibroids due to selective apoptosis of transformed cells and the extensive remodeling of the entire uterus.

Study Objectives: Monitor fibroids during pregnancy and after postpartum uterine regression to assess any loss of fibroids and change in size of fibroids.

Methods: Add a postpartum ultrasound examination to an existing epidemiologic study of pregnant women. The parent study documents fibroid number, size, and location with a 7 week ultrasound examination. With the additional postpartum ultrasound proposed here, data on fibroid number, size, and location through pregnancy and postpartum uterine regression will be collected on approximately 400 women. A subsample of 30 women will also have an MRI after their postpartum ultrasound in order to evaluate the sensitivity of ultrasound imaging.

Significance: This study will provide the first data on fibroid change with parturition/postpartum uterine regression for a large sample of women. If small fibroids disappear during this time, it will document a process that results in "natural regression" of these tumors in premenopausal women. Insights from the biology of this process may be useful in developing treatment that could be used by nonpregnant women with fibroids to induce tumor regression.