**Core Concept**
The maximum risk of developing foetal varicella syndrome (FVS) is associated with maternal varicella infection during a specific period of pregnancy, primarily due to the effect of the varicella-zoster virus on fetal development.

**Why the Correct Answer is Right**
Foetal varicella syndrome is a rare condition that occurs when a fetus is exposed to the varicella-zoster virus, primarily between 13 and 20 weeks of gestation. This period is critical because it coincides with the formation and development of vital organs and structures in the fetus. The virus can cause a range of congenital anomalies, including limb hypoplasia, cutaneous scars, and neurodevelopmental issues. The risk of FVS is highest when the maternal infection occurs during this window, as the virus can cause damage to the developing fetus.

**Why Each Wrong Option is Incorrect**
**Option A:** Infection before 13 weeks of gestation is associated with a lower risk of FVS, as the fetus is in an earlier stage of development and may be less susceptible to the virus.

**Option B:** Infection after 20 weeks of gestation is also associated with a lower risk of FVS, as the critical period of organ development has largely passed.

**Option C:** Infection during the early stages of pregnancy (before 13 weeks) may lead to miscarriage or stillbirth, but it is not the period associated with the maximum risk of FVS.

**Clinical Pearl / High-Yield Fact**
The risk of FVS can be significantly reduced if the maternal varicella infection occurs after 20 weeks of gestation.

**Correct Answer:** D.