## **Core Concept**
The most common cause of first-trimester abortion is related to chromosomal abnormalities. Chromosomal anomalies can lead to non-viable pregnancies, which often result in spontaneous abortion.

## **Why the Correct Answer is Right**
The correct answer, **C. Chromosomal abnormalities**, is right because approximately 50-60% of first-trimester abortions are due to chromosomal abnormalities in the fetus. These abnormalities can include aneuploidy (having an abnormal number of chromosomes), polyploidy (having multiple sets of chromosomes), and other genetic anomalies. These conditions usually make the embryo non-viable, leading to spontaneous abortion.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Uterine anomalies can contribute to recurrent pregnancy loss but are less commonly associated with first-trimester abortions compared to chromosomal abnormalities.
- **Option B:** Hormonal imbalances, particularly luteal phase defect, can contribute to early pregnancy loss but are not the most common cause.
- **Option D:** Infections can cause pregnancy loss but are more commonly associated with second-trimester losses or with specific conditions like chorioamnionitis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the risk of chromosomal abnormalities increases with advancing maternal age, which is a significant risk factor for first-trimester abortion. Clinicians often counsel patients about this risk, especially if there is a history of recurrent pregnancy loss.

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