**Recurrent Abortion in 1st Trimester**

**Core Concept**
Chromosomal abnormalities are a leading cause of recurrent abortion in the 1st trimester, accounting for up to 50% of cases. These abnormalities often result from errors during meiosis or mitosis, leading to aneuploid or mosaic karyotypes.

**Why the Correct Answer is Right**
Chromosomal abnormalities, such as trisomies (e.g., Trisomy 16, Trisomy 21), monosomies, or polyploidies, can lead to embryo failure, miscarriage, or recurrent pregnancy loss. The high frequency of such abnormalities in early gestation is thought to be due to the increased frequency of meiotic errors during oogenesis. The presence of chromosomal abnormalities can also trigger an immune response, making the pregnancy more vulnerable to miscarriage.

**Why Each Wrong Option is Incorrect**
**Option A:** **Hormonal Imbalance**
Hormonal imbalances, such as luteal phase defect or inadequate progesterone levels, can contribute to early pregnancy loss, but they are less common causes of recurrent abortion in the 1st trimester.

**Option B:** **Uterine Anomalies**
Uterine anomalies, such as a unicornuate or septate uterus, can increase the risk of miscarriage, but they are relatively rare and not the most common cause of recurrent abortion in the 1st trimester.

**Option C:** **Infectious Diseases**
Infectious diseases, such as chlamydia or endometritis, can increase the risk of miscarriage, but they are not typically associated with recurrent abortion in the 1st trimester.

**Clinical Pearl / High-Yield Fact**
Chromosomal abnormalities are a leading cause of recurrent abortion in the 1st trimester, emphasizing the importance of karyotyping in couples experiencing recurrent pregnancy loss.

**Correct Answer:** C. Chromosomal abnormalities are a leading cause of recurrent abortion in the 1st trimester, accounting for up to 50% of cases.