The lower segment is the lower aspect of the uterus below the utero-vesical reflection of the peritoneum. The lower segment corresponds to the isthmus in the non-pregnant uterus. The formation of the lower segment starts at approximately 26 weeks and is not completed until after 32 weeks.
The lower segment contains more fibrous tissue and less muscle fibre than the upper segment. Therefore scars in the lower segment heal strongly and are less likely to rupture in subsequent labours than scars in the upper segment (as performed in the classical caesarian section).
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