Earliest detection of pregnancy by ultrasound is by
**Core Concept**
The earliest detection of pregnancy by ultrasound is based on the visualization of the gestational sac, which is the first detectable structure in early pregnancy. This occurs due to the presence of a yolk sac and a small embryo within the sac.
**Why the Correct Answer is Right**
The earliest ultrasound detection of pregnancy, typically around 4.5-5 weeks of gestation, involves the identification of a gestational sac within the uterine cavity. The gestational sac is a fluid-filled structure that contains the developing embryo and yolk sac. The appearance of the gestational sac is characterized by a small, anechoic (black) region within the uterus. As the pregnancy advances, the embryo and yolk sac become more visible within the sac.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the corpus luteum, a structure involved in ovulation, is not directly detectable by ultrasound in early pregnancy.
**Option B:** This option is incorrect because the fetal heart activity, although detectable by ultrasound, is not the earliest sign of pregnancy.
**Option C:** This option is incorrect because the embryonic pole, a part of the embryo, is not the earliest structure detectable by ultrasound.
**Clinical Pearl / High-Yield Fact**
In early pregnancy, the presence of a gestational sac within the uterine cavity is a reliable indicator of a viable intrauterine pregnancy. This fact is essential in the assessment of early pregnancy complications, such as ectopic pregnancy.
**Correct Answer: A. Gestational sac.**