**Core Concept**
The question tests the understanding of the **teratogenic effects** of varicella zoster infection on the fetus, particularly the timing of infection that poses the greatest risk. Varicella zoster virus can cause **congenital varicella syndrome** if the mother is infected during a specific period of pregnancy.

**Why the Correct Answer is Right**
The correct answer is related to the period when the fetus is most vulnerable to the teratogenic effects of varicella zoster. Infection during the **first 20 weeks of gestation** is associated with a higher risk of congenital varicella syndrome, which can lead to severe birth defects.

**Why Each Wrong Option is Incorrect**
**Option A:** Infection after 20 weeks has a lower risk of congenital varicella syndrome.
**Option B:** Infection before conception does not directly affect the fetus.
**Option D:** Infection after birth is not relevant to congenital varicella syndrome.

**Clinical Pearl / High-Yield Fact**
It's crucial to note that varicella zoster infection during pregnancy can have serious consequences for both the mother and the fetus, and **prompt treatment with antiviral medications** may be necessary to mitigate these risks.

**Correct Answer:** B. First 20 weeks of gestation.