The risk that receives the most attention is uterine rupture — when the scar from your previous C-section opens during labor. This occurs in approximately 0.5-0.7% of VBAC attempts, or roughly 1 in 150-200 labors. While rare, uterine rupture is a medical emergency that requires immediate C-section and, in very rare cases, hysterectomy.
This is why VBAC should only be attempted at a hospital with the capability to perform an emergency cesarean within 30 minutes (the ACOG recommendation). Birthing centers and home births are not recommended for VBAC.
The other significant risk is that of a failed VBAC — laboring and then needing an unplanned C-section. An unplanned cesarean after hours of labor carries slightly higher risks (infection, blood loss) than a scheduled repeat C-section. About 20-40% of women who attempt a VBAC ultimately deliver by cesarean.