## **Core Concept**
Spontaneous abortion, also known as miscarriage, refers to the involuntary loss of a pregnancy before the fetus reaches viability, typically before 20 weeks of gestation. The etiology of spontaneous abortion is multifactorial and can involve various genetic, hormonal, anatomical, and immunological factors.

## **Why the Correct Answer is Right**
Chromosomal abnormalities are the most common cause of spontaneous abortion. These abnormalities can include aneuploidy (having an abnormal number of chromosomes), polyploidy (having multiple sets of chromosomes), or structural chromosomal anomalies. Such genetic errors usually occur during oogenesis or spermatogenesis and can lead to non-viable pregnancies. Studies have shown that approximately 50-60% of miscarriages are due to chromosomal abnormalities in the fetus.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While hormonal imbalances can contribute to difficulties in maintaining a pregnancy, they are not the most common cause of spontaneous abortion. Hormonal issues, such as luteal phase defect, can lead to miscarriage but are less frequently implicated compared to chromosomal abnormalities.
- **Option B:** Uterine anomalies can contribute to recurrent pregnancy loss but are not the most common cause of spontaneous abortion. These anomalies can include a septate uterus, bicornuate uterus, or unicornuate uterus.
- **Option C:** This option is not provided, but based on the context, we can infer that infections and autoimmune disorders are also not the most common causes.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that chromosomal abnormalities are responsible for the majority of spontaneous abortions, particularly in the first trimester. This knowledge can help guide counseling and management for patients experiencing recurrent miscarriages, where evaluation for parental chromosomal abnormalities might be warranted.

## **Correct Answer:** C. Chromosomal abnormalities.