**Core Concept**
Cordocentesis, also known as percutaneous umbilical cord blood sampling (PUBS), is a prenatal diagnostic procedure that involves sampling fetal blood from the umbilical cord. This procedure is typically performed between 18 and 22 weeks of gestation to assess fetal well-being and diagnose various fetal disorders.

**Why the Correct Answer is Right**
The primary reason for performing cordocentesis between 18 and 22 weeks is to minimize the risk of fetal loss associated with the procedure. During this period, the fetal umbilical cord is relatively longer, making it easier to access and sample blood from the cord. Additionally, the procedure is often performed to diagnose conditions such as fetal anemia, infections, and chromosomal abnormalities. The fetal blood samples obtained during cordocentesis can be analyzed for various parameters, including hemoglobin levels, infections, and chromosomal abnormalities.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because cordocentesis is not typically performed during the first trimester due to the shorter umbilical cord and increased risk of fetal loss.

* **Option B:** This option is incorrect because cordocentesis is not typically performed during the third trimester due to the increased risk of preterm labor and fetal distress associated with the procedure.

**Clinical Pearl / High-Yield Fact**
It's essential to note that cordocentesis is a relatively invasive procedure and should be performed only when absolutely necessary. The risks associated with cordocentesis include fetal loss, preterm labor, and fetal distress.

**Correct Answer: C. 18-22 weeks.**