**Core Concept:** Transvaginal Sonography (TVS) is a diagnostic imaging technique that uses high-frequency ultrasound waves to visualize internal female reproductive organs like the uterus, ovaries, and cervix. In the context of preterm labour, the cervix plays a crucial role in assessing readiness for delivery.

**Why the Correct Answer is Right:** In situations where preterm labour is suspected, TVS helps in identifying specific changes in cervix shape that indicate imminent delivery. The correct answer refers to the **shortening and internal os dilation** of the cervix. Shortening occurs due to cervical remodelling, which involves the activation of various enzymes and cytokines. This process leads to cervical softening, thinning, and dilation, eventually allowing the baby to pass through.

**Why Each Wrong Option is Incorrect:**
A. **Cervical length greater than 25mm** - This is not indicative of preterm labour, as the cervix typically shortens during labour.
B. **Cervical dilation less than 5mm** - While cervical dilation is a crucial component of preterm labour, a value less than 5mm is not specific enough to indicate imminent delivery.
C. **Cervical dilation between 5-10mm** - A dilation of this range may suggest cervical remodelling, but it is not definitive for imminent preterm labour.
D. **Cervical dilation greater than 10mm** - Although cervical dilation is a part of preterm labour, a value greater than 10mm is not indicative of imminent delivery, as it can occur in physiological processes like menstruation or postpartum changes.

**Clinical Pearl:** Preterm labour is often associated with multiple clinical signs and symptoms, including cervical changes, uterine contractions, and vaginal bleeding. In cases where these signs are present, further evaluation and close monitoring of the cervix are essential to initiate appropriate interventions and prevent complications in the mother and neonate.