The embryonic cardiovascular system grows "ahead" of the baby, stimulating new areas of development. At 6 weeks, it's possible to see the tiny embryonic heart beating. Until this week, a human embryo doesn't look very different from that of a mouse, a chicken, or a pig--but all that is about to change. Though it's only about half an inch (10-14 mm) long and weighs less than a paper clip, your baby-to-be is starting to acquire a human face. The folds of the eyelids and the jaws form, and the tip of the nose can be clearly seen. Ears are developing inside and out: internally, the semicircular canals are laid down, while externally mounds of tissue erupt where the whorls of the ears will grow. The eyes become pigmented and continue their incredibly complex development. Delicate eye muscles begin to form and nerve cells appear in the retina.

Your little embryo's posture is improving. It loosens its tight, curled-up position as the neck and trunk begin to grow and cartilage and bone elongate throughout the body. The tail begins to shrink. Fingers start to make their appearance as the hands become notched. Several days later, toe notches appear in the foot plate.