Hard to believe: though he's no bigger than a lentil, your baby already has a beating heart! By the middle of this week, his cardiac muscles have formed and begun fluttery little flexing movements. This mini -version of the human heart is your baby's first working organ.
For now, this little being still looks more like a curled-up tadpole than a human baby. A huge head makes up almost half the embryo's length (the better to accommodate a rapidly growing brain). In the midsection, near the bulge that holds the heart, tiny arm and leg buds have appeared. Just below them is the attachment point of the umbilical cord, slender as a thread. The cord connects the fetus to the growing placenta, the life-giving organ that has formed to supply the growing baby with the blood supply and nutrients needed for survival.
The body ends in a long tail. But don't worry--this reminder of man's evolutionary roots disappears within a few weeks, absorbed into the spinal cord.
Inside the body, the central nervous system, the circulatory system, and organs like the liver and pancreas are developing. Along the embryo's "back" is a row of small bony blocks called somites. These will become the spine and ribs, among other things.