**Core Concept**
The question is testing the understanding of the risks associated with organic heart disease in pregnancy. Organic heart disease refers to structural abnormalities in the heart, such as congenital heart defects or valvular heart disease. During pregnancy, the physiological changes can increase the workload on the heart, potentially leading to complications.

**Why the Correct Answer is Right**
The correct answer is **during the postpartum period**. This is because the postpartum period is a critical time for patients with organic heart disease. After delivery, the uterus contracts, which can lead to a sudden increase in blood volume and cardiac output. This can be particularly challenging for patients with pre-existing heart disease, who may not be able to compensate for the increased workload. Additionally, the postpartum period is often a time of increased stress and anxiety, which can further exacerbate cardiac symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** **During the third trimester**. While the third trimester can be a challenging time for patients with organic heart disease due to increased blood volume and cardiac output, it is not the most common time for mortality.

**Option B:** **During labor**. Labor can be a stressful time for patients with organic heart disease, but it is not typically associated with the highest risk of mortality.

**Option C:** **During the antepartum period**. The antepartum period, which includes the period before labor, is not typically associated with the highest risk of mortality for patients with organic heart disease.

**Clinical Pearl / High-Yield Fact**
Patients with organic heart disease should be closely monitored throughout pregnancy, particularly during the postpartum period. A thorough pre-conception evaluation and regular follow-up with a cardiologist and obstetrician are essential to minimize the risk of complications.

**Correct Answer:** D. During the postpartum period.