## **Core Concept**
The detection of pregnancy at 6 weeks gestation involves understanding the timeline of hormonal and physical changes that occur during early pregnancy. **Human chorionic gonadotropin (hCG)** levels start rising after implantation, which typically occurs 6-10 days after fertilization. Accurate detection methods rely on measuring hCG levels or visualizing the embryo.

## **Why the Correct Answer is Right**
The most accurate method to detect pregnancy at 6 weeks gestation is through **transvaginal ultrasound**. This method allows for the direct visualization of the gestational sac and, sometimes, the embryo's cardiac activity. At 6 weeks gestation, the gestational sac is usually large enough to be seen on a transvaginal ultrasound, which is more sensitive than a transabdominal ultrasound at this early stage. This method provides a definitive diagnosis of pregnancy and can also help in assessing the viability of the pregnancy.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, less accurate methods might include urinary hCG tests, which can be positive but do not confirm the location or viability of the pregnancy.
- **Option B:** Similarly, this option is not specified, but examples might include serum hCG tests, which, while useful, do not directly visualize the pregnancy and can sometimes be misleading due to variations in hCG levels.
- **Option C:** Without specifics, it's hard to address, but any method not directly visualizing the embryo or measuring its activity would be less accurate for confirming pregnancy at 6 weeks.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **transvaginal ultrasound can detect pregnancy as early as 4-5 weeks gestation** by identifying the gestational sac, and by 6 weeks, it can often see the embryo and its heartbeat. This timeline is crucial for early pregnancy assessment and management.

## **Correct Answer:** D. Transvaginal ultrasound.