## **Core Concept**
The first trimester of pregnancy is a critical period where various factors can lead to abortion, also known as miscarriage. The causes of first-trimester abortions can be broadly categorized into chromosomal, hormonal, anatomical, and infectious factors. Among these, chromosomal abnormalities play a significant role.

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

## **Why Each Wrong Option is Incorrect**
- **Option A:** While hormonal imbalances can contribute to pregnancy complications, they are not the most common cause of first-trimester abortions. Hormonal issues might include luteal phase defect, polycystic ovary syndrome (PCOS), or thyroid disorders.
- **Option B:** Anatomical factors, such as uterine anomalies (e.g., septate uterus, bicornuate uterus), can contribute to miscarriages but are less common causes compared to chromosomal abnormalities. These issues might lead to recurrent pregnancy loss but are not the leading cause of first-trimester abortions.
- **Option D:** Infectious causes can lead to miscarriages but are relatively rare as a primary cause of first-trimester abortions. Infections might play a role in second-trimester losses or preterm labor but are not the most common reason for early pregnancy loss.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that chromosomal abnormalities are the leading cause of first-trimester miscarriages. This fact is crucial for counseling patients who experience recurrent miscarriages, as it highlights the importance of genetic evaluation and the potential for future successful pregnancies with proper management.

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