A woman with a prior vertical classical uterine incision requests a trial of labour. Which is the key reason that uterine rupture risk is substantially higher (~8–9%) with a classical incision compared to a lower segment caesarean scar (~0.7%)?

• A Classical incision involves a more vascular segment with poor haemostatic capacity
• B The upper uterine segment (active zone) undergoes myometrial retraction and hypertrophy with each pregnancy, stressing the scar more than the passive lower segment ✓
• C Classical incisions are associated with increased chorioamnionitis risk weakening the scar
• D Lower segment scars heal by primary intention; classical scars heal by secondary intention

Explanation

The fundamental reason is anatomical and physiological: the upper uterine segment is the 'active' contracting zone that undergoes progressive thickening and retraction during labour. A classical scar in the active segment is subjected to maximum stress during uterine contractions. The lower uterine segment, where standard LSCS scars are made, is the passive zone that thins out and does not contract strongly, subjecting the scar to far less mechanical stress. This physiological difference (not vascularity or healing type) primarily explains the 10-fold higher rupture rate with classical incisions.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.