**Core Concept**
Karyotyping is a process of analyzing the number and structure of chromosomes in an individual's cells. It is a crucial diagnostic tool for identifying genetic disorders and abnormalities in the fetus. This test is usually performed on fetal cells obtained through amniocentesis, chorionic villus sampling (CVS), or other methods.

**Why the Correct Answer is Right**
Karyotyping can be performed on fetal cells obtained through various methods. Amniocentesis involves withdrawing a sample of amniotic fluid that contains fetal cells. CVS involves taking a sample of placental tissue that contains fetal cells. Fetal blood sampling, also known as cordocentesis, involves withdrawing a small sample of fetal blood. However, the correct answer is the option that does not involve fetal cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Amniocentesis is a common method for obtaining fetal cells for karyotyping. Therefore, this option is incorrect because amniocentesis is a valid method for fetal karyotyping.

**Option B:** CVS is also a valid method for obtaining fetal cells for karyotyping. This option is incorrect because CVS is a recognized technique for fetal karyotyping.

**Option C:** Fetal blood sampling, also known as cordocentesis, is another method for obtaining fetal cells for karyotyping. This option is incorrect because fetal blood sampling is a valid technique for fetal karyotyping.

**Clinical Pearl / High-Yield Fact**
It's essential to note that karyotyping is usually performed on fetal cells obtained through amniocentesis, CVS, or fetal blood sampling. These tests are typically performed between 15 and 20 weeks of gestation to detect genetic abnormalities.

**Correct Answer: D.**