**Core Concept**
Toxoplasmosis is a parasitic infection caused by *Toxoplasma gondii*, which can be transmitted vertically (from mother to fetus) during pregnancy. Vertical transmission of toxoplasmosis occurs when the parasite crosses the placenta and infects the fetus.

**Why the Correct Answer is Right**
Vertical transmission of toxoplasmosis typically occurs during the third trimester of pregnancy, particularly during the last few weeks. This is because the placenta is more developed and the fetus's immune system is less mature, making it easier for the parasite to cross the placental barrier. The parasite can also infect the fetus through the bloodstream, causing congenital toxoplasmosis.

**Why Each Wrong Option is Incorrect**
**Option A:** First trimester - While toxoplasmosis can be transmitted during any stage of pregnancy, the risk is lower during the first trimester due to the immaturity of the placenta and the fetus's immune system.
**Option B:** Early postpartum period - Toxoplasmosis is not commonly transmitted postpartum, as the mother's immune system is more effective at preventing transmission after delivery.
**Option C:** During breastfeeding - Toxoplasmosis is not typically transmitted through breast milk, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
Toxoplasmosis can be transmitted vertically during pregnancy, but the risk is lower during the first trimester. Pregnant women can reduce their risk of transmission by avoiding contact with cat feces and practicing good hygiene.

**Correct Answer:** D. Third trimester.