At week 4, your embryo is just a little ball of cells growing inside of you. It’s so tiny that it’s just the size of a poppy seed! Even with its small size, your baby now consists of three very complex layers that will form organs and tissues.

The neural tube is developing in the top layer. The brain, backbone, spinal cord, and nerves will sprout from it. The cardiovascular system, which includes the heart and blood vessels, develops from the middle layer. Finally, the urinary system, lungs, and intestines grow from the third layer.

As the embryo is rapidly developing, the chorionic villi and the umbilical cord are already working. These structures provide nutrients and oxygen to the embryo.