**Core Concept**
Cervical length is a crucial factor in assessing the risk of preterm delivery. A shorter cervical length indicates a higher risk of preterm birth due to the potential for cervical insufficiency. This is particularly relevant in the context of multiple gestations, cervical trauma, or pre-existing cervical surgery.

**Why the Correct Answer is Right**
The risk of preterm delivery is directly correlated with cervical length, as measured by transvaginal ultrasound. A cervical length of less than 25 mm is considered high-risk for preterm birth. This is because a shorter cervix is more susceptible to dilatation and effacement, leading to preterm labor. The cervix's ability to maintain its length and resist dilatation is mediated by the presence of cervical collagen and the function of the cervix's musculature.

**Why Each Wrong Option is Incorrect**
* **Option A:** A cervical length of 30 mm is generally considered within the normal range and does not increase the risk of preterm delivery.
* **Option B:** A cervical length of 20 mm is indeed at risk for preterm delivery, but the correct threshold for increased risk is less than 25 mm.
* **Option D:** A cervical length of 15 mm is extremely short and would be considered an emergency situation; however, the question asks for the threshold at which risk increases, not the threshold for an emergency situation.

**Clinical Pearl / High-Yield Fact**
Remember the "25 mm rule": a cervical length of less than 25 mm is associated with an increased risk of preterm delivery. This is a key concept in the management of high-risk pregnancies.

**Correct Answer:** C. 25 mm