For more than 20 years, it has been standard practice for doctors to use a low, side-to-side (transverse) incision across the uterus for a cesarean delivery. However, a vertical type of incision is sometimes necessary, such as for some emergency cesarean deliveries.

Before deciding whether you are a good candidate for a safe vaginal birth after cesarean (VBAC), you and your health professional must first confirm what kind of uterine incision you have had. Because the outward scar is not always positioned over the uterine scar, checking your medical record is the only dependable way to be sure of your incision type.

A low transverse (horizontal) incision:

• Cuts across the lower, thinner part of the uterus. These muscles don't contract as strongly as the upper uterus during labor.
• Is unlikely to rupture during a subsequent labor and delivery.
• Has been increasingly used for cesarean deliveries since the 1970s and is the usual practice among obstetricians.

A vertical (classical) incision:

• Cuts up and down through the uterine muscles that strongly contract during labor.
• Is more likely to break open (rupture) during a subsequent labor, particularly if the incision is high rather than low in the uterus. This risk applies to all uterine scars that are not low transverse.
• Is very rarely used for cesarean deliveries.

Uterine rupture rates

Women who have a low transverse cesarean scar have a lower risk of rupturing than women who have a vertical incision. About 5 out of every 1,000 women (0.5%) with one low transverse incision scar have a uterine rupture during labor when the labor starts on its own without medicine.¹ (It is likely that the women who rupture have other risk factors that make them more vulnerable to this complication.)

Women who have a low vertical cesarean scar (which is only on the lower uterus) are as likely to deliver vaginally as they would be with a low transverse scar. And they have no higher risk of complications, including rupture.² Some health professionals are willing to allow a woman with a very low vertical incision to try VBAC, while most are not. This is because of the concern about uterine rupture, which has only recently been refuted.

A woman with a vertical (classical) incision has the greatest uterine rupture risk. Between 4% and 9% (40 to 90 per 1,000) of women with a vertical incision develop a rupture.²