At 6 weeks of fetal development (about 4 weeks after conception), the embryo is roughly the size of a lentil, measuring about 4–6 millimeters long. The body is curved into a C-shape, with a large head area forming in proportion to the rest of the body. The embryo is enclosed within the amniotic sac and connected to the yolk sac, which provides early nourishment. One of the most significant milestones at this stage is the beginning of cardiac activity. The primitive heart tube has formed and star