**Core Concept:**
Anti-D immune globulin is a specific preparation of polyvalent immunoglobulins, containing anti-D antibodies, used to prevent Rh-D hemolytic disease in Rh-negative mothers who are pregnant with an Rh-positive fetus. The primary purpose is to prevent the formation of anti-D antibodies in the mother, which can cross the placenta and cause hemolysis in the Rh-positive fetus.

**Why the Correct Answer is Right:**
The correct answer, D, is "at the first antenatal visit." This is because anti-D immune globulin is administered as prophylactic therapy to Rh-negative mothers to prevent the formation of maternal anti-D antibodies before the fetus can cause hemolysis in the baby. By administering it earliest (at the first antenatal visit), the mother receives the treatment as early as possible, ensuring a higher likelihood of preventing the production of anti-D antibodies and reducing the risk of hemolytic disease in the fetus.

**Why Each Wrong Option is Incorrect:**
A: Administering it at 28 weeks gestation is too late. By this stage, the mother may have already developed anti-D antibodies, and administering the therapy will not prevent hemolytic disease.
B: Administering it at 37 weeks gestation is also too late. Even though the mother has not developed anti-D antibodies, this late intervention does not guarantee the prevention of hemolytic disease in the fetus.
C: Administering it at 12 weeks gestation is too early. The fetus has not yet formed Rh antibodies, and administering the therapy at this stage is not necessary and could cause adverse reactions in the mother due to the administration of foreign antibodies.

**Clinical Pearl:**
The primary purpose of administering anti-D immune globulin is to prevent the formation of anti-D antibodies in Rh-negative mothers who are pregnant with an Rh-positive fetus. To minimize risks and ensure maximum efficacy, administer the therapy as early as possible during pregnancy, ideally at the first antenatal visit. This timely intervention helps to prevent the development of anti-D antibodies in the mother, reducing the risk of hemolytic disease in the Rh-positive fetus.