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There is no sure way to predict which patients will suffer uterinerupture if they attempt VBAC. U/S, however, holds promise forevaluating factors that may suggest increased risk.
How to manage delivery in the woman who previously gave birth via cesarean? That is the subject of much controversy in our specialty, even as cesarean rates rise despite the federal government's attempts to lower them.1 Much effort has gone into trying to identify risk factors for uterine rupture so that we can predict which patients are likely to have a successful vaginal birth after cesarean (VBAC).2 With our country's increasing cesarean rates, too, have come increased risks of placenta previa and accreta for women in subsequent pregnancies.3-5 Ultrasound has proved useful in helping to settle other obstetric controversies. Might it also have a role in assessing uterine scars before pregnancy and predicting risk of uterine rupture? Let's examine the evidence.
Are scars significant?
In early studies of uterine scars, transabdominal U/S was used to distinguish between classical and transverse uterine incisions. More recent efforts with transvaginal U/S have shown that it is more effective for such screening.6
In another study using SIS, Regnard and colleagues investigated the frequency of sonographic dehiscence in women with a history of cesarean delivery. They saw a "niche" in 57.5% of patients (Figure 1), and in 6% of patients, a "deep niche" extending through 80% of the myometrium was visible.8
There have been reports describing repair of uterine defects detected by U/S. One case report describes the repair of a uterine dehiscence in pregnancy detected at 28 weeks' gestation with subsequent conservative management and elective delivery at 35 weeks.9 Another recent series describes the laparoscopic and vaginal repair of uterine defects detected by U/S in five nongravid women. One woman in this series went on to have an uncomplicated, term repeat C/S.10
Whether these findings-and the depth of the niche--are clinically significant remains unknown. What we do know, however, is that conventional U/S and SIS can be used to identify uterine scars in women who have delivered by cesarean, before they become pregnant again. Several sonographic findings, including "deep niches," are suggestive of a uterine wall defect, but it's unclear whether they have clinical significance or are related to subsequent uterine rupture.
Intrapartum uterine rupture during a trial of labor after prior cesarean is rare, occurring in about 0.5% of women, when cervical ripening with oxytocin or prostaglandin is not required.11 Uterine niches, in contrast, are seen in 6% to 42% of women who have delivered by cesarean, making them too common to show a clear relationship between a niche and uterine rupture. Their clinical significance and relationship to adverse pregnancy outcomes also have yet to be determined. At this time, repair of uterine niches is considered experimental.
What about scars at term?
U/S also has been used by clinicians to diagnose uterine rupture before the onset of labor, and recently, researchers have tried to predict which women may be at increased risk of uterine rupture.