What is a common sign of pregnancy observable via ultrasound?

The presence of the gestational sac is a fundamental and early sign of pregnancy that can be visualized through ultrasound imaging. The gestational sac appears as a fluid-filled structure within the uterus and serves as the first visible indication of an ongoing pregnancy, typically detectable around 4 to 5 weeks of gestation. Its presence confirms that the embryo is developing within the uterus rather than ectopically, which is critical for evaluating the viability and health of the pregnancy.

In contrast, although increased amniotic fluid levels may occur later in pregnancy, it is not a primary or first observable sign of pregnancy. Measurement of maternal blood flow is more relevant for assessing the well-being of the fetus and the placenta rather than indicating pregnancy itself. The identification of maternal organ development might be evident as the pregnancy progresses, but it is not a direct or early sign of pregnancy visible via ultrasound. Therefore, the gestational sac provides the clearest and most immediate evidence of pregnancy observable through ultrasound early on.