Fetal Development at 4 Weeks: What Can Be Seen?

At four weeks, an embryo is very small—just a little over a millimeter long—but it is already starting to look like a person. The following features can be seen on a four-week-old fetus:

* A rudimentary head: The head is bent down and makes up about half the length of the embryo; the brain is beginning to develop, but the eyes are not yet visible.

* Early stages of the heart and lungs: The heart is a simple tube, which will start beating in the next week. The lungs are still developing and will only be fully formed by the end of the pregnancy.

* Small arm and leg buds: These will start developing into arms and legs in the coming weeks.

* A tail: This is a remnant of the embryo's evolutionary ancestors. It will disappear before birth.

At this stage, the embryo is completely dependent on the mother for survival. It is still embedded in the uterine lining and receives oxygen and nutrients from the mother's bloodstream through the placenta.