By this time your baby looks like a flat flower petal. It consists of three cell layers. They are, the topmost layer (ectoderm), middle layer (mesoderm) and the bottommost layer (endoderm).The ectoderm thickens further and forms a narrow tube which lies along what would later be your baby’s back. The front end of this tube expands and starts to form the baby’s brain. Rest of the tube start to form the baby’s spinal cord and the peripheral nerves.
Remember that the brain and the nervous system does not form within one week. It is a very long process that extends well beyond day 28. Brain activity also starts for the first time during the fourth week. Your baby’s smell sensors and eyes start their long development process.
During the fourth week the endoderm forms the primitive gut. Out-pocketing from this layer begins formation of the large airways and liver. Hormone secreting glands like the pituitary and the thyroid begin development during the fourth week. Your baby’s heart start out as a twisted tube and starts beating in the fourth week. You baby’s heart is the first ever functioning organ.
The middle layer commits itself to form the skeleton, muscles, soft tissue, kidneys and internal reproductive organs of your baby. Right now, none of these organs are there. For now your baby appears to have two rows of bulges (somites) on either side of its midline. The number of somites coincide with your baby’s age.
By this time you know that you are pregnant, or you suspect so because of you are late in getting your periods. These days are the happiest of your life. Apart from the happiness you do not feel any different physically. Your loved ones try to pamper you. You start to think about what you would need later to make your baby happy.
Right now your baby does not demand much. Just only a bit of your energy. Follow a healthy diet, stick to a regular moderate exercise routine. And above all be happy. Avoid any unhealthy behaviors. Start reading up a bit on your upcoming joys and challenges. Parent crafting would be a good study for new parents.