Which parameter is critical in determining the viability of a pregnancy during an ultrasound?

The presence of a fetal heart rate is a crucial parameter in determining the viability of a pregnancy during an ultrasound because it is a direct indicator of fetal well-being and development. A detectable fetal heartbeat typically suggests that the embryo is developing normally and has a higher likelihood of being in a viable pregnancy. The fetal heart rate can usually be seen as early as six weeks of gestation with transvaginal ultrasound, making it a key metric for assessing the health of the pregnancy.

Other measurements, such as the presence of the gestational sac, while important, do not provide the same level of assurance regarding the viability of the embryo. Similarly, the size of the placenta and the distance to the cervix are parameters that may provide additional information about the pregnancy's progression, but do not directly indicate the viability of the fetus in the same immediate way that the presence of a heartbeat does.