**Core Concept**
Second trimester abortion, also known as second trimester miscarriage, is a pregnancy loss that occurs between 13 and 26 weeks of gestation. The most common causes of second trimester abortion involve a combination of hormonal and mechanical factors that compromise the integrity of the pregnancy.

**Why the Correct Answer is Right**
The most common cause of second trimester abortion is **placental abruption**, where the placenta separates from the uterus. This can lead to a decrease in blood flow to the fetus, resulting in fetal distress and ultimately, abortion. Placental abruption can be caused by various factors, including hypertension, trauma, and uterine distension. The release of thromboxane A2 and prostaglandins can also contribute to the separation of the placenta from the uterus.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chromosomal abnormalities are a common cause of first trimester abortion, not second trimester abortion.
* **Option B:** Uterine anomalies can cause pregnancy loss, but they are not the most common cause of second trimester abortion.
* **Option C:** Intrauterine infection can cause second trimester abortion, but it is not the most common cause.

**Clinical Pearl / High-Yield Fact**
Placental abruption is often associated with a history of hypertension, trauma, or uterine distension. It is essential to investigate and manage these underlying factors to prevent second trimester abortion.

**Correct Answer: A. Placental abruption.