Uterine rupture is potentially very dangerous, however, still very rare. It can also occur in the non-scarred pregnant uterus in all three trimesters. (SEE: Title: Spontaneous rupture of the unscarred uterus.
Author: Sweeten KM; Graves WK; Athanassiou A
Address: Department of Obstetrics, Gynecology, and Reproductive Sciences, University of Texas Health Science Center at Houston, USA.
Source: Am J Obstet Gynecol, 172: 6, 1995 Jun, 1851-5).

From OB/GYN Secrets by Wilkins-Haug & Fredrickson:

Section 77 VBAC by Robert Silver MD

"2. What is uterine rupture?

To accurately asses the risk of VBAC, it is necessary to differentiate between complete or true uterine rupture and incomplete rupture, often termed occult ruptue or uterine dehiscence. True uterine rupture is often sudden and associated with pain, blood loss and fetal morbidity. It is most commonly seen in spontaneous or traumatic rupture of the unscarred uterus. It also has been associated wiht classic uterine scars, ofen occuring without labor. Conversely, uterine dehiscence in partial separation of the uterine wall that is usually asymptomatic and rarely contributes to fetal or maternal morbidity. This is often the type of separation seen in lower segment scars, and usually occurs during labor. Ocften asymptomatic windows are incidentally noted at the time of repeat cesarean section.

3. Is there a difference in the risks of uterine dihiscence with classic vs. lower segment uterine scars?

There is little difference in the rate of uterine dihiscence between the scar types, approximately 1-2% for both. However, the data are difficult to interpret. Many studies fail to distinguish between rupture and dehiscence. Further clouding the data, several studies that examined the uterus after delivery (either postpartum or at the time of a cesarean section) revealed higher dehiscence rates than studies including only symptomatic cases. Incidence ranges from 0.4 - 4% for both types. However, there is a large difference in the morbidity from these different types of scar separation. In reviews prior to 1987, encompassing almost 10,000 VBACs, there were no maternal deaths from rupture of either scar type. Of 20 fetal deaths, 17 were from classical scars. Of the three from lower segment rupture, all occurred over 20 years years ago in unmonitored patients. These data conclude that "rupture" of a lower transverse uterine scar rarely carries significant fetal morbidity. As such, VBAC is recommended for patients with previos lower segement incision. In addition, classic scars frequently separate before delivery, whereas lower segment scars tend to separate during delivery. "

Title
Trial of labor after cesarean delivery with a lower-segment, vertical uterine incision: is it safe?

Author
Naef RW 3rd; Ray MA; Chauhan SP; Roach H; Blake PG; Martin JN Jr

Address
Department of Obstetrics and Gynecology, University of Mississippi Medical Center, Jackson 39216-4505, USA.

Source
Am J Obstet Gynecol, 172: 6, 1995 Jun, 1666-73; discussion 1673-4

Abstract

OBJECTIVE: Our purpose was to assess maternal and perinatal outcomes associated with a trial of labor and attempted vaginal birth after prior low-segment vertical cesarean delivery. STUDY DESIGN: During a 10-year period in a single tertiary hospital, all patients with a prior low-segment uterine incision (whether vertical or transverse) were considered candidates for a trial of labor in the absence of other contraindications or patient refusal. Among the 1137 women who underwent low-segment vertical cesarean delivery, 262 were subsequently delivered of 322 live-born infants, and 174 (54%) of them were identified retrospectively as having attempted vaginal birth. The maternal and perinatal outcomes of patients who did or did not undergo a trial of labor were analyzed and compared. RESULTS: No significant differences between the two patient groups were observed regarding demographic characteristics, antepartum complications, gestational age at delivery (mean 37.4 weeks), birth weight, and cord pH at delivery. Vaginal delivery was accomplished successfully in 144 of 174 (83%) patients who underwent a trial of labor. Abdominal delivery was necessary for 17 mothers with labor disorders and 13 with suspected fetal distress. Postpartum hemorrhage occurred more often in the trial of labor group (7/174 [4.0%] vs 2/148 [1.4%], p not significant), but endometritis developed significantly more often in patients with elective repeat cesarean delivery (16.9% vs 6.3%, p = 0.006). Rupture of the low-segment vertical cesarean scar occurred in 2 patients during a trial of labor (1.1%) versus none in the elective repeat cesarean group. Neither mother experienced fetal extrusion or adverse maternal or fetal sequelae. Frequency of serious neonatal complications (8.1% vs 10%) and neonatal mortality (1.7% vs 2.0%) were similar between groups. All neonatal deaths were a result of extreme prematurity or congenital anomalies. CONCLUSIONS: Our experience indicates that a mother with a prior low-segment vertical cesarean delivery can undertake a trial of labor with relative maternal-perinatal safety. The likelihood of successful outcome and the incidence of complications are comparable to those of published experience with a trial of labor after a previous low-segment transverse incision.