Newsletter from Essential Baby - week 5

What happens after the blastocyst implants?

The blastocyst, which is your developing baby, is now called an embryo. By week 5, the inner cell mass of the blastocyst has flattened to form the embryonic disc and the body stalk. During this week, the inner cell mass (now the embryonic disc) further differentiates into three basic germ layers - ectoderm, mesoderm, and endoderm. Each of these layers is responsible for the development of different parts of your baby's body.
The ectoderm (outer layer) will provide the entire nervous system and brain, the skin, hair, glands, eyes and ears. The mesoderm (middle layer) will form the supporting structures such as the bones and joints, muscles, and connective tissue, together with the heart and kidneys and the vascular, genital, and urinary systems. Finally, the endoderm (inner layer) is responsible for the gastrointestinal tract, including the liver, gall bladder and pancreas, and the lungs. This week the major developmental activity occurs in the nervous system and heart.
This week the beginnings of the brain and spinal cord are formed. This occurs when 2 ridges form the neural canal which continues to grow until (at the end of the week) the ridges meet and fuse, forming an open-ended tube in the region that will become the back of your baby's head.
How big is my baby?

At the start of week 5, your baby hasn't grown much larger - the embryo is only about 1.25mm (0.05 inches) long.