Which pregnancy signs can be detected using an ultrasound?

Ultrasound is a key diagnostic tool in obstetrics that allows for the visualization of the developing fetus during pregnancy. This imaging technique is particularly useful for confirming the presence of a fetus in the uterus, assessing its growth and development, and determining other important factors such as gestational age and the position of the fetus.

The ability to visualize the fetus using ultrasound is a definitive sign of pregnancy and provides direct evidence of fetal presence, which cannot be established through other methods such as hormone tests or physical signs. The other options listed relate to signs or test results that do not provide visual confirmation of fetal development. For instance, a positive urine test indicates hormonal changes associated with pregnancy, Chadwick's sign involves changes in skin color, and nausea may suggest hormonal changes as well, but none of these options offer direct visual confirmation that an ultrasound can provide. Thus, the capability of ultrasound to visualize the fetus makes it the correct answer.