**Question:** Commonest cause of first trimester miscarriage is:

A. Fetal chromosomal abnormalities
B. Immune-mediated response
C. Placental insufficiency
D. Uterine anomalies

**Core Concept:** The first trimester of pregnancy is typically defined as weeks 1-12, representing the crucial period for early fetal development. Miscarriages in this phase are of significant clinical relevance.

**Why the Correct Answer is Right:** The commonest cause of first trimester miscarriage is fetal chromosomal abnormalities (Option A). Chromosomal abnormalities occur when the number of chromosomes in the developing fetus does not match with that of the mother, leading to developmental anomalies and miscarriage.

**Why Each Wrong Option is Incorrect:**

A. Immune-mediated response (Option B) is not the primary cause of miscarriage in the first trimester, although it can contribute in certain cases. This option is incorrect because the primary cause is chromosomal abnormalities.

C. Placental insufficiency (Option C) usually occurs later in pregnancy, leading to preterm labor or intrauterine growth restriction, rather than first trimester miscarriage.

D. Uterine anomalies (Option D) can contribute to miscarriage in specific cases, but it is not the primary cause. The primary reason is chromosomal abnormalities in the fetus.

**Clinical Pearl:** While immune-mediated response, placental insufficiency, and uterine anomalies can contribute to miscarriages, chromosomal abnormalities in the fetus are the most common cause in the first trimester. Understanding the underlying causes can help healthcare professionals identify potential complications and provide appropriate management or counseling to patients.