Others should chime in here, but according to the literature this is seriously rare.
Some quick info below (http://emedicine.medscape.com/articl...ew#aw2aab6b3):
Rupture of the Unscarred Uterus
The normal, unscarred uterus is least susceptible to rupture. Grand multiparity, neglected labor, malpresentation, breech extraction, and uterine instrumentation are all predisposing factors for uterine rupture. A 10-year Irish study by Gardeil et al showed that the overall rate of unscarred uterine rupture during pregnancy was 1 per 30,764 deliveries (0.0033%). No cases of uterine rupture occurred among 21,998 primigravidas, and only 2 (0.0051%) occurred among 39,529 multigravidas with no uterine scar.
A meta-analysis of 8 large, modern (1975-2009) studies from industrialized countries revealed 174 uterine ruptures among 1,467,534 deliveries. This finding suggested that the modern rate of unscarred uterine rupture during pregnancy is 0.012% (1 of 8,434). This rate of spontaneous uterine rupture has not changed appreciably over the last 40 years, and most of these events occur at term and during labor. An 8-fold increased incidence of uterine rupture of 0.11% (1 in 920) has been noted in developing countries. This increased incidence of uterine rupture has been attributed to a higher-than-average incidence of neglected and obstructed labor due to inadequate access to medical care. When one assesses the risk of uterine rupture, this baseline rate of pregnancy-related uterine rupture is a benchmark that must be used as a point of reference.
1. Gardeil F, Daly S, Turner MJ. Uterine rupture in pregnancy reviewed. Eur J Obstet Gynecol Reprod Biol
. Aug 1994;56(2):107-10.