What is the first definitive sign of an intrauterine pregnancy (IUP) detected via ultrasound?

The presence of an intrauterine gestational sac is considered the first definitive sign of an intrauterine pregnancy (IUP) detected via ultrasound. During early pregnancy, this gestational sac forms as a fluid-filled structure within the uterus and is the initial indicator that a pregnancy is located inside the uterus rather than outside or in an ectopic location.

The gestational sac can typically be visualized as early as 4-5 weeks of gestation, and its identification helps confirm that the pregnancy is progressing normally within the uterine cavity. Once the gestational sac is detected, further developments such as the yolk sac and eventually fetal cardiac activity can be observed, but the gestational sac is the primary marker of an established intrauterine pregnancy.

Other options, like the yolk sac or cardiac activity, indicate further development within the IUP, but they are not the first signs. The decidual reaction also signals a reaction of the endometrial lining to the pregnancy but occurs concurrently with the formation of these structures rather than serving as the definitive sign of an intrauterine pregnancy itself.