The fertilized egg, which is now a ball of cells, will continue to move down your fallopian tube until it reaches your uterus. It will stay there for approximately three days before it embeds itself in your uterine lining. This is implantation. During implantation the fertilized egg can attach to the lining of your uterus and begin to receive protection, oxygen and nutrients from your bloodstream.
Your baby-to-be is still just 0.014 to 0.04 inches in size (0.36mm to 1mm). Though your future baby is still minuscule, great developments are being made. The placenta and vascular networks are forming. Germ layers that develop into specialized parts of the body, such as organs are also developing. Cells continue to divide. Some cells will become the placenta and others will form the amniotic sac where the baby will develop. Other cells will become the yolk sac which is responsible for producing blood corpuscles. In spite of the rapid cell division that is taking place, your baby is so tiny right now that he or she is barely visible. This is the week that your baby will transform from a blastocyst (a ball of cells) into an embryo.
The embryo at this time becomes differentiated into three different tissue-types: The inner layer, called the endoderm, will form the lungs, liver, digestive system and pancreas. The middle layer, called the mesoderm, will become the skeleton, muscles, kidneys, blood vessels and heart. The outer layer, called the ectoderm, will become the skin, hair, eye lenses, tooth enamel and nervous system. The cells from each type of tissue will move around to the appropriate location. The embryo is shaped like a teardrop.
Implantation is complete. Your future baby has now reached beneath the lining of your uterus and growth is rapid. Your baby is now called an embryo. The embryo is still about 0.05 inches (1.25mm) long. It is deeply embedded in your uterus.
It is now possible to identify the fold of skin that will become your baby's head. A heart forming plate has developed and the central nervous system, muscles and bone are in the early stages of development. Your baby's heart has started beating. The brain, spine and beginnings of the nervous system are forming.
By the end of this month, your baby will resemble a tiny tadpole, barely the size of a grain of rice. A distinct organization, however, is becoming visible. There is a stripe down the back of the embryo which is curling over to form a groove which will seal up to form the neural tube. This will turn into the spinal cord and brain. The top of the tube is flattening out and expanding to form the front part of the brain.
The placenta, which will provide nourishment to your developing baby later in your pregnancy, is not yet formed. Right now your baby is nourished from you via hundreds of tiny "roots" that cover the amniotic sac. The nourishment is channeled from you to your baby from these roots through a stalk that will become the umbilical cord. The placenta will begin its development this week. It will form in the site that the fertilized egg attached to the lining of your uterus at the time of implantation. A bulge is developing where your baby's heart will be and the blood vessels are growing into place.
Source: http://preg.fertilityfriend.com