At 17 weeks, a pregnancy is in its second trimester; most fetal organs and structures are present and the fetus looks very much like a miniature baby. But there's still plenty of growth and refinement to take place before a fetus is ready to be born. At this stage of pregnancy, a fetus can't yet survive outside the womb.
When the embryo first implants in the womb around the third week of pregnancy, it resembles a ball of cells. But within that ball are three layers that will grow into different parts of the fetus: the ectoderm, or outside layer; the mesoderm, or middle layer and the endoderm, or outside layer. All the genetic material to create the fetus at 17 weeks are already present.
The fetus at 17 weeks measures around 9 inches long, 5 inches from crown to rump and weighs about 5 ounces. While this may not seem like much, it's a far cry from the embryo that was the size of a pinpoint just 13 weeks before.
In many ways, the 17-week fetus looks quite complete. Arms, legs, fingers and toes are developed. The eyes have developed to the point where the retina is sensitive to light. But the eyelids are still sealed and will be until around the 22nd week. The skin is very thin, and blood vessels are visible beneath the surface; the hair patterns have formed. The fetus can grimace, frown and squint. He moves freely and often, although the motions still can't be felt. The ears move away from the head and are starting to hear. The sex of the fetus can be distinguished.
By 17 weeks, the fetal heart is formed and is pumping up to 25 quarts of blood a day since week 16, the American Pregnancy Association states. Meconium, the first stool, has begun to form in the intestines. Fat begins to accumulate under the skin during the 17th week. Sucking and swallowing skills are put to the test recycling amniotic fluid. Bone is starting to form in place of cartilage.
Although the major organs are in place and the fetus appears developed, she is not yet viable outside the womb. The lungs are not yet able to exchange gases, but the bronchioles, the small airways, are developing as the lungs enter the canalicular stage during the 17th week, Brown University states.