Your baby's heart is growing and dividing into the right and left chambers. But the most exciting part is that her tiny heart is now beating (at a speedy 100 to 130 beats a minute) and pumping blood throughout her body. The cells that will make up all of your baby's body parts and systems are dividing furiously as her body begins to take shape. Right now she's about the size of a small lentil bean (4 to 5 millimeters across).
If you could see through your uterine wall, you'd find an overlarge head and dark spots where your baby's eyes and nostrils will be. Shallow pits on the sides of her head mark her developing ears, and her arms and legs appear as protruding buds. Her hands and feet look like paddles, with thick webbing between the developing digits, but her fingers and toes will soon become more distinct. Below the opening that will later be your baby's mouth, there are small folds where her neck and lower jaw will eventually develop. (Inside, her tongue and vocal cords are just beginning to form.) Your baby's intestines are developing, and tiny breathing passages are beginning to appear where her lungs will be. She's also starting to build muscle fibers and, halfway through this week, she'll likely start moving her tiny limbs.
This is one cool picture: http://www.babycenter.com/general/fetaldevelopment/pregnancy/1498210.html?scid=mbtw_preg07:20070319:0:0:0