The effect of fibroids — benign growths on the uterine wall — on pregnancy depends on their size and location. But fibroids may affect pregnancy in several ways. Fibroids may increase the risk of preterm labor and preterm birth. Additionally, they may affect the fetus' growth and lead to a small baby. A small percentage of women with fibroids (5 to 15 percent) may experience significant pain associated with the fibroids due to a process called degeneration. This can happen when fibroids enlarge due to pregnancy-related hormones, outgrowing their blood supply and becoming painful. But, the good news is that the majority of women with fibroids experience no problems during pregnancy.