Vaginal birth after cesarean section
The cesarean section rate has
skyrocketed in recent years to 29% in the USA at last count.
Asthe primary cesarean section rate rises, so does the repeat cesarean section rate, since many patients and theirproviders are unwilling to try for a vaginal birth after cesarean section (VBAC). However, a recent study shows
that over 75% of attempted VBACs will succeed and therisk of uterine rapture is low. Macones et al. [1] reviewed
4 years worth of data from 11 centers in the United Statesand found that the uterine rupture rate in VBACs was
about 1%. In those who had asuccessful vaginal birthprior to the VBAC, the uterine rupture rate is closerto 0.5%.
Recent attention has been directed to identifying patients with cesarean scars who are at greater risk of uterine
rupture. Two studies that have shed important light onthis subject have focused on the width of the uterine wall
in the vicinity of the uterine scar. In a study from Japan[2] the authors evaluated the lower uterine segment wallthickness just under the bladder flap in patients about tohave attempted VBACs. They found that if the thinnest
diameter was 2 mm or less, there was a 30% risk of uterinerupture. Conversely, if the diameter exceeded 2.9 mm, no
uterine ruptures occurred.
In another study [3] involving 642 patients, the sametechnique was used to assess the wall thickness before
attempted VBAC. The unique wrinkle in this study isthat the ultrasound examinations were undertaken by a
single practitioner. The results indicated that the criticalcut-off point for uterine rupture was a wall thickness of
3.5 mm, below which there was a 20-fold greater risk foruterine rupture and above which there was a 99% nega-
tive predictive value for uterine rupture. If the wall thickness was between 1.6 and 2.5 mm, a scar defect ultimately
was found in 16% of these patients. Later, they used the3.5 mm cut-off in a prospective trial in which managing
physicians were given the uterine wall information and,although those with thin measurements were sectioned
more frequently, the overall uterine rupture rate decreasedappreciably.