**Core Concept**
The upper segment to lower segment (US:LS) ratio is a measure used to assess fetal growth and development, particularly in the context of growth restriction or other fetal anomalies. This ratio is calculated by dividing the distance from the crown of the head to the top of the symphysis pubis (upper segment) by the distance from the top of the symphysis pubis to the heel (lower segment).

**Why the Correct Answer is Right**
The US:LS ratio becomes 1:1 at approximately 15-16 weeks of gestation. This is because, at this stage, the fetal head and body are roughly equal in size, resulting in a balanced ratio. Prior to this, the fetal head is relatively larger, and the ratio is higher. As the pregnancy progresses, the body grows more rapidly than the head, and the ratio decreases.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the US:LS ratio is not 1:1 at birth, which is a common misconception.
**Option B:** This is incorrect because the US:LS ratio becomes 1:1 before 16 weeks of gestation, not after.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the US:LS ratio is a dynamic measurement that changes throughout pregnancy, and any significant deviations from the expected ratio may indicate fetal growth restriction or other underlying issues.

**Correct Answer:** C. 15-16 weeks.