## **Core Concept**
The detection of cardiac motion in an embryo is an important milestone in early pregnancy assessment. It is a critical indicator of embryonic viability. The ability to visualize cardiac motion depends on the resolution of the ultrasound technology and the size of the embryo.

## **Why the Correct Answer is Right**
The correct answer, , indicates that cardiac motion can be visible with transvaginal ultrasound when the embryo length has reached 5 mm. Transvaginal ultrasound provides higher resolution images than transabdominal ultrasound, especially in early pregnancy, due to its proximity to the uterus. At an embryonic length of 5 mm, which typically corresponds to around 5-6 weeks of gestation, the cardiac activity can be detected. This is because the embryonic heart starts to beat and pump blood very early in development, and by this stage, it is sufficiently large to be visualized with high-resolution ultrasound.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This length is too short for reliable detection of cardiac motion with transvaginal ultrasound. At 1 mm, the embryo is usually around 3-4 weeks post-conception, and cardiac activity might not be visible yet.
- **Option B:** While 3 mm is a plausible size for some early embryonic structures, it's generally still a bit early for consistent visualization of cardiac motion.
- **Option D:** This option suggests a much larger size than necessary for the detection of cardiac motion. By the time the embryo reaches 10 mm, cardiac motion is easily visible, but this is well beyond the threshold for detection.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the earliest detection of cardiac activity is an important prognostic indicator of pregnancy viability. The American College of Obstetricians and Gynecologists (ACOG) and other guidelines often reference the detection of cardiac activity as a critical milestone. A common mnemonic or fact to recall is that with modern transvaginal ultrasound, cardiac activity can typically be seen by the time the crown-rump length (CRL) of the embryo reaches about 5 mm, which usually corresponds to approximately 5-6 weeks of gestation.

**Correct Answer: B. 5 mm.**