Updated December 01, 2014.
Fibroids (leiomyomata) are typically non-cancerous tumors that grow in the uterus. They can grow in the interior or exterior of the uterus. Typically these slow growing tumors don't get bigger than golf ball size but they can get to be as big as a watermelon and weigh as much as 50 pounds.
There has been a fear that a fibroid can interfere with a vaginal birth, meaning a cesarean section would be needed.
This is not the case. Studies showed that labors are usually the same length and the vaginal birth rates aren't different. Studies also showed that there are probably more fibroids in pregnancy than previously suspected, but that this didn't adversely effect labor or birth outcomes.
"In general, the literature tends to underestimate the prevalence of fibroids in pregnancy and overestimate the complications that are attributed to them," claims the abstract of "Obstetric complications of Fibroids. Even when the leiomyomata (fibroids) were 10 centimeters or great, the vaginal birth rate was still 70.
Fibroids are usually found by ultrasound for other reasons or are known about prior to pregnancy. Typically they do not grow much in pregnancy, other than in the first trimester. Typically no treatment is needed even after birth. However, surgical removal immediately following birth was found to carry a higher risk of postpartum hemorrhage.
Sources:
Obstetric complications of fibroids. Ouyang DW, Economy KE, Norwitz ER. Obstet Gynecol Clin North Am. 2006 Mar;33(1):153-69.
Obstetric outcomes in women with sonographically identified uterine leiomyomata. Qidwai GI, Caughey AB, Jacoby AF. Obstet Gynecol. 2006 Feb;107(2 Pt 1):376-82.