Title: Embryonic Period | The first weeks after fertilization
Headings: Embryonic Period
Embryonic Period
The first weeks after fertilization
Fertilization
Week 1
Week 2
Weeks 3 to 8

Content: Weeks 3 to 8
In the third week of embryogenesis, the embryo begins to develop a brain, heart and spinal cord. The beginnings of a gastrointestinal system first appear. In weeks four and five, arms and legs first begin to bud, as do the structures that will eventually become eyes and ears. Brain development continues, the heart beats at regular intervals and the first bones begin to form. In week seven, all essential organs have begun their development and fingers, toes, nipples and hair follicles begin to form. In week eight, the ears and facial features are clearly visible. In most cases, the fetus now measures in at about an inch in length and weighs under an ounce. Early limb movements and brain waves can be recorded at this time. The embryonic stage of fetal development ceases at eight weeks from the moment of fertilization. At this point, embryogenesis is complete and the embryo has developed into an organism that we begin to call a fetus.