**Core Concept**
The fetal varicella syndrome is a condition that occurs when a fetus is exposed to the varicella-zoster virus (VZV) in utero. The risk and severity of the syndrome are influenced by the timing of maternal infection during pregnancy.

**Why the Correct Answer is Right**
Maternal VZV infection during the first trimester, particularly between 13 and 20 weeks of gestation, poses the highest risk of fetal varicella syndrome. This is because the virus can cause teratogenic effects during the critical periods of organogenesis, leading to congenital abnormalities. The severity and likelihood of fetal varicella syndrome increase with the proximity to the 20th week of gestation.

**Why Each Wrong Option is Incorrect**
**Option A:** Infection during the second trimester (22-26 weeks) carries a lower risk of fetal varicella syndrome compared to the first trimester.
**Option B:** Infection during the third trimester (after 36 weeks) typically does not result in fetal varicella syndrome, as the fetus has developed protective mechanisms against the virus.
**Option C:** Infection before conception or early in pregnancy (before 8 weeks) poses minimal risk of fetal varicella syndrome, as the embryo has not yet undergone organogenesis.

**Clinical Pearl / High-Yield Fact**
Fetal varicella syndrome is characterized by a range of congenital anomalies, including limb hypoplasia, skin scarring, and neurological deficits. A high index of suspicion should be maintained for pregnant women presenting with varicella infection, particularly during the first and early second trimesters.

**Correct Answer:** C. 20 weeks of gestation