## **Core Concept**
The detection of pregnancy through transvaginal ultrasound (TVS) is closely related to the levels of beta-human chorionic gonadotropin (β-hCG) and the sensitivity of the ultrasound technology. β-hCG is a hormone produced during pregnancy that is made by the developing embryo after conception and later by the placental component syncytiotrophoblast.

## **Why the Correct Answer is Right**
The correct answer, **1000-2000 IU/L**, is significant because at this level of β-hCG, a pregnancy can typically be visualized using transvaginal ultrasound. This is because at β-hCG levels around 1000-2000 IU/L, the gestational sac is usually large enough (about 2-3 mm in diameter) to be detectable by TVS. The sensitivity of modern transvaginal ultrasound probes allows for the detection of smaller gestational structures compared to transabdominal ultrasound, enabling earlier detection of pregnancy.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Levels below 100 IU/L are generally too low for detection of pregnancy via ultrasound, as the embryo and gestational sac are too small.
- **Option B:** While some studies suggest pregnancy can be detected at β-hCG levels as low as 500 IU/L with very sensitive equipment, 500-1000 IU/L is still on the lower side for reliable detection by standard TVS.
- **Option D:** Levels above 5000 IU/L would typically be associated with a more advanced pregnancy, easily detectable by both transvaginal and transabdominal ultrasound.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the timing of β-hCG level measurement and ultrasound detection can vary based on factors like the sensitivity of the ultrasound machine and the skill of the operator. Generally, a β-hCG level of 1000-2000 IU/L is a practical threshold for expecting to see a gestational sac on a transvaginal ultrasound.

## **Correct Answer:** . **1000-2000 IU/L**