The embryo is so small in this fourth week that it is not possible to see it by ultrasound. The first structure detected in an ultrasound is the gestational sac, which usually measures about 2 millimeters (mm) in diameter in week 4 + 2 and usually grows 1 mm per day.
Sometimes, it is also possible to visualize the yolk sac, a small sphere filled with fluid that is inside the gestational sac. Although the yolk sac is detected before the embryo in an ultrasound scan, its presence is positive, as it rules out a possible anembryonic pregnancy.
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