## **Core Concept**
The most common cause of first-trimester abortion is related to chromosomal abnormalities in the fetus. These abnormalities can lead to non-viable pregnancies, which often result in spontaneous abortion. The etiology of first-trimester abortions is multifactorial but is predominantly linked to genetic factors.

## **Why the Correct Answer is Right**
Chromosomal abnormalities, particularly aneuploidy (having an abnormal number of chromosomes), are the most common cause of first-trimester abortion. About 50-60% of first-trimester miscarriages are due to chromosomal abnormalities in the fetus, such as monosomy X (45,X), trisomy 16, and triploidy. These abnormalities usually occur due to errors during meiosis. The presence of such chromosomal abnormalities often leads to early pregnancy loss because the fetus is not viable.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While uterine anomalies can contribute to recurrent pregnancy loss, they are not the most common cause of first-trimester abortion. Uterine factors might play a role in second-trimester losses or recurrent miscarriages but are less frequently associated with first-trimester abortions.
- **Option B:** Hormonal imbalances, particularly luteal phase defect, can contribute to early pregnancy loss. However, they are not as commonly associated with first-trimester abortion as chromosomal abnormalities.
- **Option D:** Infections can cause pregnancy loss but are more commonly associated with second-trimester losses or preterm labor rather than first-trimester abortions.

## **Clinical Pearl / High-Yield Fact**
A significant clinical pearl is that most first-trimester miscarriages are due to chromosomal abnormalities, which are usually non-recurrent. Therefore, after experiencing a first-trimester miscarriage due to chromosomal abnormalities, the likelihood of a subsequent pregnancy being affected is similar to that of the general population.

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