**Question:** Rh isoimmunisation in which trimester

**Core Concept:** Rh isoimmunisation is a condition that occurs when a pregnant woman's Rh-negative blood is exposed to Rh-positive blood from the fetus, leading to the mother's immune system producing anti-D antibodies. These antibodies can cross the placenta and enter the fetal circulation, causing Rh isoimmunisation.

**Why the Correct Answer is Right:** Rh isoimmunisation mainly occurs during the later stages of pregnancy, specifically in the third trimester. This is when the Rh-negative mother's immune system starts producing anti-D antibodies in response to the Rh-positive fetal blood. These antibodies can cross the placenta and enter the fetal circulation, potentially causing severe Rh-related hemolytic disease (e.g., Rh hemolytic disease of the newborn) if left untreated.

**Why Each Wrong Option is Incorrect:**

A. The second trimester (Option A) is too early for the production of anti-D antibodies, as the immune system does not start recognizing the fetal Rh factor until the third trimester.
B. The first trimester (Option B) is too early for Rh isoimmunisation to occur, as the mother's immune response is not yet activated.
C. While Rh isoimmunisation can occur in both the first and second trimesters, the third trimester (Option C) is more specific to the scenario when the mother's immune system is actively producing anti-D antibodies and the fetus is Rh-positive.
D. The timing of Rh isoimmunisation is crucial in understanding the subsequent hemolytic disease of the newborn, which primarily affects the third trimester (Option D).

**Correct Answer:**

**D.** The third trimester (Option D) is the correct answer because it is during this period that the mother's immune system becomes reactive to Rh-positive antigens in the fetal blood, leading to the production of anti-D antibodies and potentially causing severe Rh-related hemolytic disease of the newborn if left untreated.