## **Core Concept**
Fetal sex determination via ultrasound (USG) relies on the visualization of fetal genitalia, which becomes discernible during the second trimester. The accuracy of sex determination depends on the gestational age, fetal position, and the skill of the operator.

## **Why the Correct Answer is Right**
The correct answer, , indicates that fetal sex can be detected by USG at 16 weeks. At this gestational age, the external genitalia are sufficiently developed, making it possible to identify the sex of the fetus with a reasonable degree of accuracy. The presence of testes or ovaries and the development of the genital tubercle into either a penis or clitoris allow for sex determination.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because, at 10 weeks, the external genitalia are not fully developed, making it difficult or impossible to accurately determine the fetal sex via USG.
* **Option B:** This option is incorrect because, although 14 weeks is a possible timeframe for determining fetal sex, it might still be early for accurate determination in all cases.
* **Option D:** This option is incorrect because, while 20 weeks is certainly a timeframe when fetal sex can be determined, it is not the earliest gestational age provided as an option.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the accuracy of fetal sex determination via USG increases with gestational age, particularly after 16 weeks. Factors influencing accuracy include fetal position, the presence of conditions like ambiguous genitalia, and the skill of the ultrasound operator.

## **Correct Answer:** . 16 weeks.