**Core Concept**
Transplacental transmission of Toxoplasma gondii refers to the vertical transmission of the parasite from mother to fetus through the placenta. The risk of transmission is influenced by various factors, including the timing of maternal infection, the parasite load, and the immune status of the mother.

**Why the Correct Answer is Right**
The risk of transplacental transmission of Toxoplasma gondii is maximum when the mother is infected during the first trimester of pregnancy. This is because the parasite is more likely to cross the placenta and infect the fetus during this critical period of organogenesis. The parasite can invade the placenta and cause congenital toxoplasmosis, which can lead to severe complications in the fetus, including hydrocephalus, chorioretinitis, and intellectual disability.

**Why Each Wrong Option is Incorrect**
* **Option A:** Infection during the third trimester is associated with a lower risk of transmission because the placenta is more mature and less likely to allow the parasite to cross.
* **Option B:** Infection during the second trimester is associated with a moderate risk of transmission, but not as high as infection during the first trimester.
* **Option C:** Infection during the fourth trimester is not a typical time frame for pregnancy, and even if it were, the risk of transmission would be lower than during the first trimester.

**Clinical Pearl / High-Yield Fact**
To minimize the risk of transplacental transmission, pregnant women should be advised to avoid undercooked meat, unwashed fruits and vegetables, and cat feces, which can contain Toxoplasma gondii oocysts.

**Correct Answer:** D.