**Core Concept**
Recurrent abortion in the 1st trimester is a complex condition with multiple potential causes. It involves the failure of a pregnancy to progress beyond 20-24 weeks of gestation, resulting in recurrent miscarriages. This condition is often associated with underlying hormonal, anatomical, or immunological abnormalities.

**Why the Correct Answer is Right**
The most common cause of recurrent abortion in the 1st trimester is **anembryonic pregnancy** (also known as blighted ovum). This occurs when there is a failure of proper embryonic development, leading to the absence or underdevelopment of the embryo. It is often due to a chromosomal abnormality in the embryo, which makes it non-viable.

**Why Each Wrong Option is Incorrect**
* **Option A:** Uterine anomalies are a possible cause of recurrent abortion, but they are less common than anembryonic pregnancy. Uterine anomalies can lead to an increased risk of miscarriage, but they are not the most common cause.
* **Option B:** Thrombophilias (blood clotting disorders) can increase the risk of recurrent abortion, but they are not the most common cause. Thrombophilias can lead to clotting in the placenta, which can compromise fetal development and lead to miscarriage.
* **Option C:** Chromosomal abnormalities in the mother are a possible cause of recurrent abortion, but they are not the most common cause. Chromosomal abnormalities in the mother can increase the risk of miscarriage, but they are often associated with other symptoms or complications.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the majority of recurrent abortions in the 1st trimester are due to anembryonic pregnancy, which is often associated with chromosomal abnormalities in the embryo.

**Correct Answer: A. Anembryonic pregnancy**