Which factor most increases the risk of uterine rupture in a 9‑week obese twin pregnancy with a prior classical (upper‑segment) uterine incision 8 months ago: twin gestation, obesity, prior classical uterine incision, or short inter‑pregnancy interval?

The Prior Classical (Upper Uterine Segment) Incision is the Dominant Risk Factor

The prior classical uterine incision is by far the most significant risk factor for uterine rupture in this pregnancy, carrying a 1-12% rupture risk compared to 0.22-0.35% with a low transverse scar. 1, 2, 3

Why the Classical Incision Dominates All Other Risk Factors

Magnitude of Risk from Classical Incision

• Classical (upper segment) uterine incisions carry a 1-12% risk of uterine rupture in subsequent pregnancies, even without labor 2, 3
• The incidence of uterine rupture following classical incision without attempting labor is 1% 3
• This represents a 10-100 fold increase compared to the baseline uterine rupture rate of 3.3 per 10,000 births (0.033%) in the general population 4
• Classical uterine scars are located in the contractile portion of the uterus, making them far more vulnerable to rupture as the pregnancy progresses 4

Why Other Factors Are Secondary

Short interpregnancy interval (8 months):

• Inter-delivery interval <18 months does increase rupture risk, but this applies primarily to low transverse scars attempting vaginal birth after cesarean (VBAC) 1, 5
• The baseline rupture risk with short interval is approximately 1.1% during labor with oxytocin augmentation 1
• This pales in comparison to the 1-12% risk from the classical incision alone 2

Twin gestation:

• Twin pregnancy causes uterine overdistension, which theoretically increases rupture risk 6
• However, no high-quality evidence quantifies twin gestation as an independent risk factor for uterine rupture in the absence of prior uterine surgery 7, 8
• The primary concern with twins is malpresentation and operative delivery, not spontaneous rupture 7

Obesity:

• Obesity increases surgical morbidity and wound complications but is not established as a direct risk factor for uterine rupture 4
• No guidelines or studies in the evidence base identify obesity as increasing rupture risk 4

Critical Clinical Implications

The Classical Incision Makes This a High-Risk Pregnancy

• Classical uterine scar is an absolute contraindication to trial of labor 1, 9, 5
• This patient must have a scheduled repeat cesarean delivery, ideally at 36-37 weeks gestation to minimize rupture risk while optimizing neonatal outcomes 2
• The facility must have immediate cesarean capability and blood products available, as rupture can occur before labor 1, 2, 3

Time-Sensitive Nature of Rupture

• If uterine rupture occurs, delivery must occur within 18 minutes to prevent permanent neonatal injury 1
• Outcomes deteriorate significantly if delivery occurs >30 minutes after suspected rupture 1, 5
• The most consistent early indicator is prolonged, persistent fetal bradycardia 7

Common Pitfall to Avoid

Do not be distracted by the multiple risk factors present. While the short interval, twins, and obesity all contribute to overall pregnancy risk, the classical incision is the single factor that fundamentally changes management and carries the highest rupture risk 1, 9, 5, 2. The other factors are clinically relevant for counseling about surgical complications and pregnancy monitoring, but the classical scar is what makes this pregnancy immediately high-risk for catastrophic uterine rupture 4, 2, 3.

Answer: C - Upper uterine segment (classical) incision