Timing of first appearance of gestational landmarks on transvaginal ultrasound examination

The yolk sac is visible when the mean gestational sac diameter (MSD) is 8 mm and fetal cardiac activity can be observed when MSD is 16 mm. For transabdominal sonograms, the corresponding MSDs are larger than 20 and 25 mm, respectively. MSD = (length + height + width of the gestational sac)/3. In addition, MSD(mm)+30 = gestational age(days).