Earliest ultrasound sign of pregnancy in a transabdominal ultrasound scan is :
**Core Concept**
The earliest ultrasound sign of pregnancy is an essential diagnostic tool in obstetrics, allowing for the confirmation of pregnancy and assessment of fetal development. The sign is based on the visualization of a specific anatomic structure within the uterus.
**Why the Correct Answer is Right**
The earliest ultrasound sign of pregnancy is the presence of a gestational sac, which is a fluid-filled structure that forms around the embryo. The gestational sac can be visualized as early as 4.5-5 weeks of gestation, making it the earliest reliable indicator of pregnancy. The sac is seen as a small, anechoic structure within the uterus, and its presence is a reliable indicator of a viable intrauterine pregnancy.
**Why Each Wrong Option is Incorrect**
**Option A:** The yolk sac is a structure that develops within the gestational sac and is not the earliest sign of pregnancy.
**Option B:** The embryo itself is not visible on ultrasound until around 5-6 weeks of gestation, making it a later sign of pregnancy.
**Option C:** The decidua capsularis is a layer of tissue that forms within the uterus in response to pregnancy, but it is not a reliable indicator of pregnancy and can be seen in non-pregnant women as well.
**Clinical Pearl / High-Yield Fact**
The presence of a gestational sac within the uterus is a reliable indicator of a viable intrauterine pregnancy, and its absence is associated with a high risk of miscarriage or ectopic pregnancy.
**Correct Answer:** C. Decidua capsularis