**Question:** Fetal cardiac activity can be detected by vaginal USG by how many weeks after LMP?

**Core Concept:**
The detection of fetal cardiac activity is a crucial aspect of fetal assessment, and it is typically used as an indicator of fetal well-being. Vaginal ultrasound (USG) is a common method used to assess fetal development and overall health, including the detection of cardiac activity.

**Why the Correct Answer is Right:**
Fetal cardiac activity can be detected using vaginal USG after about **18 weeks** of gestation, which is calculated from the last menstrual period (LMP). This is due to the increased size and visibility of the fetus, which allows for better visualization of the heart and subsequent detection of cardiac activity.

**Why Each Wrong Option is Incorrect:**
A. **16 weeks:** While it is possible to sometimes detect cardiac activity earlier, 16 weeks is too early, as the fetus is too small for accurate visualization and assessment.
B. **20 weeks:** Although fetal cardiac activity can be more easily visualized at 20 weeks, 18 weeks is considered the minimum time frame for reliable detection using vaginal USG.
C. **22 weeks:** Similar to option B, 22 weeks is too late, as the earlier detection allows for better monitoring of fetal health and growth.
D. **24 weeks:** This option is incorrect as the fetus becomes more visible and cardiac activity can be detected earlier, at around 18 weeks, using vaginal USG.

**Clinical Pearl:**
The ability to detect fetal cardiac activity by 18 weeks after LMP allows for more comprehensive fetal monitoring, including assessment of fetal well-being, growth, and development. Detecting cardiac activity earlier can help identify potential issues that may arise during pregnancy, potentially leading to earlier interventions and improved maternal and fetal outcomes.

**Correct Answer:** 18 weeks