## **Core Concept**
The prenatal screening for Down's syndrome involves assessing the risk of a fetus having Down syndrome (Trisomy 21) through various markers. The screening tests are typically performed during specific periods of pregnancy to identify potential risks.

## **Why the Correct Answer is Right**
The correct answer, , is considered the best time for prenatal screening of Down's syndrome because it aligns with the period when the combined screening test (which includes the first-trimester screening and the second-trimester screening) provides the most accurate results. The first-trimester screening, performed between 11 and 14 weeks, measures the thickness of the nuchal translucency (NT) and assesses the levels of certain substances in the maternal blood, such as free beta-human chorionic gonadotropin (β-hCG) and pregnancy-associated plasma protein-A (PAPP-A). The second-trimester screening, usually done between 15 and 20 weeks, evaluates the levels of alpha-fetoprotein (AFP), β-hCG, estriol, and inhibin A. Combining these results provides a more accurate risk assessment for Down's syndrome.

## **Why Each Wrong Option is Incorrect**
- **Option A:** is too early for comprehensive screening since it only allows for the first-trimester screening, which, although useful, does not provide a complete picture on its own.
- **Option B:** is too late for the optimal combined screening, as it misses the window for the first-trimester screening.
- **Option D:** is also not ideal because waiting until after 20 weeks may delay diagnosis and options for parents.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the first-trimester screening (between 11-14 weeks) is crucial for assessing the risk of Down syndrome, and when combined with second-trimester markers, it significantly increases the detection rate. Non-invasive prenatal testing (NIPT) can also be performed early in pregnancy, often after 9-10 weeks, but it is not a diagnostic test, only a screening tool.

## **Correct Answer: B. .**