**Core Concept:** HIV (Human Immunodeficiency Virus) testing is a crucial component of preventive healthcare, particularly in high-risk populations like pregnant women. The Centers for Disease Control and Prevention (CDC) provides guidelines on when and how often to screen for HIV to ensure early detection and intervention, ultimately improving patient outcomes and reducing the risk of onward transmission.

**Why the Correct Answer is Right:** According to the CDC, pregnant women should undergo HIV screening at two specific points during their pregnancy:

1. **First prenatal visit:** This ensures early detection and initiation of antiretroviral therapy (ART) to reduce maternal-to-child transmission. ART is a combination of medications that suppress the virus and boost the immune system, thus decreasing the risk of onward transmission to the baby.
2. **At labor and delivery:** HIV testing in this context is crucial to identify HIV-positive women and prevent perinatal transmission by administering appropriate prophylaxis to the newborn.

**Why Each Wrong Option is Incorrect:**

A. **Not conducting screening at all:** This option disregards the importance of early detection, ART initiation, and the subsequent reduction of HIV transmission to the baby.
B. **Only screening at childbirth:** Early screening during the first prenatal visit allows for ART initiation, further reducing the risk of vertical transmission. Screening at childbirth alone may result in delayed treatment and increased transmission risk.
C. **Only screening at termination of pregnancy:** HIV testing should occur as early as possible to allow for ART initiation, preventing HIV transmission to the baby. Screening late in pregnancy does not offer sufficient time for intervention to reduce transmission risk.
D. **Screening at postpartum period:** HIV testing during the postpartum period is insufficient as it misses the critical window for ART initiation and prevention of perinatal transmission.

**Clinical Pearl:** Early HIV screening during pregnancy, along with ART initiation, is essential for optimizing maternal health outcomes and reducing perinatal transmission risk. Timely intervention enables effective antiretroviral therapy, promoting maternal immune system function and decreasing the probability of onward transmission to the baby.