**Core Concept**
Rubella infection during pregnancy poses a significant risk to the fetus, particularly during the first trimester. This is due to the teratogenic effects of the virus on fetal development. The virus can cause congenital rubella syndrome (CRS), which is a spectrum of congenital anomalies.

**Why the Correct Answer is Right**
The risk of CRS is highest when the maternal infection occurs during the first trimester, specifically between 8 and 16 weeks of gestation. During this period, the fetus is most susceptible to the teratogenic effects of the virus. The virus can infect the placenta and cause damage to the developing fetus, leading to a range of congenital anomalies, including hearing loss, visual impairment, and heart defects.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while the risk of CRS is still present during the second trimester, it is significantly lower than during the first trimester.
**Option B:** This option is incorrect because the risk of CRS decreases significantly after the 16th week of gestation.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the risk of CRS can be significantly reduced with vaccination, especially in women of childbearing age. The rubella vaccine is a live, attenuated vaccine and is contraindicated in pregnant women.

**Correct Answer: C.**