Week 5

This week, the cluster of cells that had begun to look like a round, solid mass, have started to grow lengthwise and the embryo now develops a head as well as a tail end. This is the stage where the central nervous system begins to develop by forming a niche in the top layer that extends to the tail end of the embryo. The cells fold up to create a hollow neural tube that form your baby's brain and spinal cord. Simultaneously, the beginnings of what will be the spine, ribs and abdominal muscles are being created out of developing tissue. 

Even though the baby's nervous system, spine and brain are being formed, and as you know they are the major component parts, your little one is really still very little. He or she is just about one tenth of an inch. That is 2mm to be precise. But relax, you are on the right track, as I am sure your urine test will have told you by now.