**Question:** Risk of preterm delivery is increased if cervical length is lesser than:

**Core Concept:** Cervical length is an important parameter in assessing a pregnant woman's risk of preterm birth. A shorter cervical length indicates a higher risk of premature labor.

**Why the Correct Answer is Right:** Cervical length is a measure of the distance from the internal os (opening) of the cervix to the external os. A shorter cervical length is associated with a higher risk of preterm labor because it may indicate a less mature cervix that is more susceptible to changes that lead to premature contractions and delivery.

**Why Each Wrong Option is Incorrect:**

A. **25 mm:** This answer option is incorrect because a cervical length of 25 mm is not the minimum value that increases the risk of preterm delivery. A shorter cervical length is more concerning.

B. **20 mm:** This answer option is incorrect because a cervical length of 20 mm is not the minimum value that increases the risk of preterm delivery. A shorter cervical length is more concerning.

C. **15 mm:** This answer option is incorrect because a cervical length of 15 mm is not the minimum value that increases the risk of preterm delivery. A shorter cervical length is more concerning.

D. **10 mm:** This answer option is correct because a cervical length of 10 mm is considered a threshold below which the risk of preterm delivery increases significantly. A cervical length of 10 mm or less is often used as a clinical cutoff to monitor and manage patients for preterm labor.

**Clinical Pearl:** Monitoring cervical length is a part of the routine ultrasound assessment during pregnancy, particularly in high-risk patients. A decrease in cervical length can indicate an increased risk for preterm labor, and interventions like cerclage placement or steroid administration may be considered to prevent preterm delivery.