## Core Concept
Toxoplasmosis is a parasitic infection caused by *Toxoplasma gondii*, which can be transmitted from mother to fetus during pregnancy. The risk of transmission varies throughout gestation.

## Why the Correct Answer is Right
The highest rate of transmission of toxoplasmosis in pregnancy occurs during the third trimester. However, the severity of the infection is inversely related to the gestational age at the time of transmission. Early in pregnancy, transmission rates are lower, but the risk of severe congenital toxoplasmosis is higher. By the third trimester, the transmission rate peaks, but the disease tends to be less severe. The correct answer, third trimester, reflects the period with the highest transmission rate.

## Why Each Wrong Option is Incorrect
* **Option A:** First trimester - This option is incorrect because, although the risk of severe congenital toxoplasmosis is higher when infection occurs early in pregnancy, the transmission rate is not the highest during this period.
* **Option B:** Second trimester - This option is incorrect because, while there is a significant risk of transmission during the second trimester, it is not the period with the highest transmission rate.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the risk of transmission of *Toxoplasma gondii* increases as pregnancy advances, but the severity of congenital toxoplasmosis decreases. Pregnant women should be counseled about avoiding cat feces and undercooked meat to reduce the risk of toxoplasmosis.

## Correct Answer: C. Third trimester.