Which of the following conditions is the most frequent cause of spontaneous aboion in the first trimester of pregnancy?
**Core Concept**
Chromosomal abnormalities are a leading cause of early pregnancy loss, often due to errors in meiosis or mitosis during the formation of gametes or embryoblasts. The most common genetic cause of spontaneous abortion in the first trimester is a chromosomal anomaly.
**Why the Correct Answer is Right**
The most frequent cause of spontaneous abortion in the first trimester is aneuploidy, particularly trisomy 16, trisomy 13, trisomy 18, and trisomy 21. These chromosomal abnormalities occur due to non-disjunction during meiosis, resulting in an abnormal number of chromosomes in the zygote. The fetus is often unable to survive the developmental processes, leading to early miscarriage.
**Why Each Wrong Option is Incorrect**
* **Option A:** Cervical incompetence is a leading cause of second-trimester pregnancy loss, not first-trimester losses.
* **Option B:** Uterine anomalies can contribute to recurrent miscarriages but are not the most common cause of first-trimester spontaneous abortion.
* **Option D:** Infections can cause pregnancy loss, but they are less common as a cause of first-trimester spontaneous abortion compared to chromosomal abnormalities.
**Clinical Pearl / High-Yield Fact**
A significant proportion of early pregnancy losses are due to chromosomal abnormalities, with an estimated 50-60% of first-trimester miscarriages being aneuploid. This highlights the importance of genetic counseling for women experiencing recurrent miscarriages.
**Correct Answer: B. Uterine anomalies can contribute to recurrent miscarriages but are not the most common cause of first-trimester spontaneous abortion.