By the end of this week, your baby is likely about the size of the tip of a pen.
Your baby is considered a “blastocyst”; by the time it reaches the uterus, the rapidly dividing ball of cells — right after conception was known as a zygote, but now already known as a blastocyst — has separated into two sections, and has implanted into your uterine wall. It is the inner group of cells that is considered your actual baby. The outer group of cells have begun to form the membranes of the placenta. It is the detection of this microscopic, very early placenta which triggers hormonal changes and a positive pregnancy test.
In week 5, the inner group of cells, your actual baby, has divided into 3 layers; each of these layers are the foundation for particular body development.