Implantation began the first week and continues. By the end of the first month, the embryo has a head and a trunk. Structures that will become arms and legs, called limb buds, begin to appear. The heart, now in a tubular form, begins to beat by the 25th day. The embryo is about half an inch long and has a four-chambered heart.
Electrical activity begins in the developing brain and nervous system.
The fingers begin to develop. The embryo has nostrils. The head is about half the size of the embryo. The beginnings of all key body parts are present, although they are not completely developed or positioned in their final locations. The embryo has ears, fingers, and toes.