## **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**, corresponds to the level of β-hCG at which a normal pregnancy can be earliest detected by TVS. At this concentration, the embryo is usually large enough (about 2-3 mm in diameter) and the gestational sac can be visualized using transvaginal ultrasound. This typically occurs around 4-5 weeks of gestation. The sensitivity of modern TVS allows for the detection of pregnancies at an earlier gestational age compared to transabdominal ultrasound.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Levels below 100 IU/L are generally too low for the detection of pregnancy via ultrasound, as the embryo would be too small.
- **Option B:** While levels around 500-800 IU/L might be getting closer to the detectable range, they are still on the lower side for reliable detection of a pregnancy via TVS.
- **Option D:** Levels above 5000 IU/L would correspond to a later stage of pregnancy, far beyond the earliest detection point.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the gestational age is calculated from the first day of the last menstrual period (LMP), which can be about 2 weeks before actual conception. Therefore, by the time a pregnancy test is positive and an ultrasound can detect the pregnancy, the patient is approximately 4-5 weeks pregnant. The β-hCG levels can vary widely among individuals, but in general, a level of 1000-2000 IU/L is a good threshold for expecting to see a gestational sac on a transvaginal ultrasound.

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