**Core Concept**
Recurrent abortion refers to the occurrence of three or more consecutive spontaneous abortions before 20 weeks of gestation. Various factors contribute to recurrent abortion, including hormonal, anatomical, and immunological abnormalities.

**Why the Correct Answer is Right**
The correct answer is **Thyroid disorders**. Thyroid abnormalities, particularly hypothyroidism, can lead to recurrent abortion due to its impact on the endometrium, ovulation, and embryonic implantation. The thyroid hormone plays a crucial role in the regulation of the menstrual cycle, and its deficiency can disrupt the normal functioning of the reproductive system. Studies have shown that women with thyroid disorders are at an increased risk of recurrent abortion.

**Why Each Wrong Option is Incorrect**
**Option A:** **Antiphospholipid syndrome** is a known cause of recurrent abortion. This condition is characterized by the presence of antiphospholipid antibodies, which can lead to thrombosis and placental insufficiency, resulting in recurrent pregnancy loss.

**Option B:** **Uterine anomalies**, such as a unicornuate or bicornuate uterus, can also lead to recurrent abortion. These anomalies can disrupt the normal development of the embryo and increase the risk of miscarriage.

**Option C:** **Diabetes mellitus** is another known cause of recurrent abortion. Uncontrolled diabetes can lead to vascular complications and placental insufficiency, resulting in recurrent pregnancy loss.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that thyroid disorders are a common cause of recurrent abortion, and thyroid function tests should be included in the evaluation of women with recurrent pregnancy loss.

**Correct Answer:** D. Thyroid disorders.