Embryo at 39 days: Above left, we see the embryo and sac--just 2.6 mm in diameter (1/4 cm)--very small. The black is the gestational sac. The yellow highlighted "barbell" shaped dot in the lower right is the 5.5 week embryo and yolk sac. Right is a picture of the uterus at top (dark gray) with gestational sac in the intrauterine cavity. The two very small white lines along the right edge of the gestational sac represent the 5.5 week old embryo. Above right is an enlarged view of the gestational sac with the embryo on the right.