**Core Concept**
During pregnancy, HIV transmission from mother to child can occur at various stages, but the risk varies depending on the gestational age and the mode of delivery. Understanding the timing of HIV transmission is crucial for implementing effective interventions to prevent mother-to-child transmission (MTCT) of HIV.

**Why the Correct Answer is Right**
The risk of HIV transmission from mother to child is highest during the intrapartum period, particularly during the third trimester and at the time of delivery. This is because the infant is exposed to the mother's viremic state, and the fetal immune system is not yet fully developed to combat the virus. The intrapartum transmission occurs through the placenta during labor and delivery, and the risk is higher in the presence of maternal viremia, prolonged rupture of membranes, and cesarean delivery without labor.

**Why Each Wrong Option is Incorrect**
**Option A:** Intrauterine transmission occurs during the second and third trimesters, but the risk is lower compared to the intrapartum period. Fetal exposure to maternal HIV during this time is limited, and the risk of transmission can be reduced with antiretroviral therapy.
**Option B:** Postpartum transmission occurs after delivery, and the risk is lower compared to the intrapartum period. Breastfeeding is a significant risk factor for postpartum transmission, and exclusive formula feeding can significantly reduce this risk.
**Option C:** Fetal transmission occurs during the second and third trimesters, but the risk is lower compared to the intrapartum period. Fetal exposure to maternal HIV during this time is limited, and the risk of transmission can be reduced with antiretroviral therapy.

**Clinical Pearl / High-Yield Fact**
Antiretroviral therapy (ART) is effective in reducing the risk of mother-to-child transmission (MTCT) of HIV. ART should be initiated as soon as possible in pregnancy, and the infant should receive ART for 6 weeks postpartum to prevent postpartum transmission.

**Correct Answer:** C.