**Core Concept:** Cordiocentesis is a procedure performed during fetal blood sampling, where a needle is inserted into the umbilical cord to collect blood from the fetal circulation. This is done to obtain fetal blood for laboratory analysis in cases of suspected genetic disorders or infections.

**Why the Correct Answer is Right:** Cordiocentesis is typically done in the **3rd trimester** of pregnancy, usually between 24-28 weeks, when the risk for fetal survival is higher and the procedure has a lower risk of complications for the fetus.

**Why Each Wrong Option is Incorrect:**
1. **Option A:** Cordiocentesis is not performed in the **1st trimester** as it poses significant risk to the developing fetus.
2. **Option B:** While cordocentesis can be performed in the **2nd trimester**, the optimal time for the procedure is during the 3rd trimester, as mentioned earlier.
3. **Option C:** The procedure is not performed in the **postnatal period** as it is exclusively done during pregnancy to assess fetal health.

**Clinical Pearl:** The optimal timing for cordocentesis in the 3rd trimester is crucial to minimize fetal risk while ensuring accurate diagnostic results. This knowledge is essential for healthcare providers dealing with high-risk pregnancies, as it helps guide clinical decision-making and patient management.