## **Core Concept**
The most common cause of spontaneous abortion in the first trimester is related to chromosomal abnormalities. Spontaneous abortion, also known as miscarriage, is a pregnancy that ends on its own before the fetus can survive outside the uterus. The first trimester is a critical period for fetal development, and various factors can lead to miscarriage during this time.

## **Why the Correct Answer is Right**
The correct answer, chromosomal abnormalities, accounts for approximately 50-60% of first-trimester miscarriages. These abnormalities can include aneuploidy (having an abnormal number of chromosomes), which often results from nondisjunction during meiosis. This process leads to embryos with lethal genetic errors that are incompatible with continued development. The most common chromosomal abnormalities include monosomy X (45,X), trisomy 16, and triploidy. These genetic errors usually occur sporadically and are not inherited.

## **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 spontaneous abortion in the first trimester. Uterine factors are more commonly associated with second-trimester losses.
- **Option B:** Hormonal imbalances, particularly luteal phase defect, can contribute to early pregnancy loss but are less common causes compared to chromosomal abnormalities.
- **Option D:** Infections can cause miscarriage 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 most first-trimester miscarriages due to chromosomal abnormalities are sporadic events and do not significantly increase the risk of recurrence in future pregnancies. However, recurrent miscarriages (three or more consecutive losses) may warrant further investigation for parental chromosomal abnormalities, uterine anomalies, or other causes.

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