**Core Concept:** Transvaginal ultrasound (TVS) is a type of ultrasound examination performed using a transducer inserted into the vagina. In pregnancy, TVS can detect early changes in the endometrium and embryo/fetus. The early sign of pregnancy on TVS is an endometrial thickness increase.

**Why the Correct Answer is Right:** The earliest sign of pregnancy on TVS is the increase in endometrial thickness. In non-pregnant women, the endometrium has a uniform thickness of around 4-5 mm. During pregnancy, the endometrial thickness increases due to the hormonal influence of progesterone, which stimulates endometrial growth. This increase in endometrial thickness helps differentiate pregnancy from other conditions like endometritis or endometrial hyperplasia.

**Why Each Wrong Option is Incorrect:**

A. **Endometrial polyp:** An endometrial polyp is a benign growth within the uterine cavity, but it is not a pregnancy sign; it does not increase in thickness with pregnancy.

B. **Endometrial hyperplasia:** Endometrial hyperplasia refers to excessive endometrial growth due to various factors, not specifically pregnancy. It may increase in thickness but is not a pregnancy sign.

C. **Endometrial cancer:** Endometrial cancer is a malignant growth of endometrial cells and can increase in thickness. However, it is not specific to pregnancy and lacks other typical pregnancy signs, making it less reliable.

D. **Endometrial cyst:** An endometrial cyst is a fluid-filled sac in the endometrium. While it may increase in size, it is not a pregnancy sign and lacks typical pregnancy features.

**Clinical Pearl:** The increase in endometrial thickness is a crucial early sign of pregnancy on TVS, allowing for an early diagnosis and monitoring of pregnancy progress.