**Core Concept**
External ballottement is a clinical maneuver used to assess the presence and position of a fetus in the uterus. It involves a gentle palpation of the uterus to detect the movement of the fetus as it floats in the amniotic fluid. This maneuver is essential in obstetrics to determine the gestational age and evaluate fetal positioning.

**Why the Correct Answer is Right**
External ballottement can be performed after 20 weeks of gestation when the uterus is large enough to allow the fetus to move freely within the amniotic fluid. At this stage, the fetus is large enough to be detected by ballottement, which involves a gentle pressure on the uterine wall to displace the fetus and assess its movement. The movement of the fetus is a sign of its viability and well-being.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because external ballottement cannot be performed before 20 weeks of gestation when the uterus is too small and the fetus is not large enough to be detected.

**Option B:** This option is incorrect because external ballottement cannot be performed after 36 weeks of gestation when the fetus is too large and the movement is restricted by the uterine contractions and the decrease in amniotic fluid volume.

**Option C:** This option is incorrect because external ballottement cannot be performed after 40 weeks of gestation when the fetus is in a fixed position and the movement is restricted by the uterine contractions and the decrease in amniotic fluid volume.

**Clinical Pearl / High-Yield Fact**
External ballottement is a crucial clinical maneuver in obstetrics that helps determine the gestational age and evaluate fetal positioning. It is essential to perform this maneuver gently to avoid causing fetal distress or discomfort.

**Correct Answer: D. 20 weeks**