During weeks five through eight of your pregnancy your baby’s cells multiply rapidly and they begin to perform specific functions. This process of specialization is called differentiation. It’s necessary to produce all the different cells that make up a human being. As a result of differentiation, your baby’s main external features also will begin to take shape. No longer just a mass of cells, your baby - now officially called an embryo - is starting to take on a distinct form.
Now that you're 5 weeks pregnant, the embryo has divided into three layers. In the top layer, a groove develops and then closes to form the neural tube, which will eventually develop into your baby’s brain, spinal cord, spinal nerves and backbone. This groove runs along the 5 weeks pregnantmid-line of the body, from the top to the bottom of the embryo. The closure of the neural tube begins in the embryo’s midsection. It proceeds upward and downward from there, like a double zipper. The top portion thickens to begin forming the brain.
From the embryo’s middle layer of cells, the heart and the circulatory system are taking shape. A bulge at the center of the embryo will develop into your baby' s heart. By week’s 5 end, the earliest blood elements and blood vessels have formed, both in the embryo and the developing placenta.
Your baby’s first heartbeats occur at 21 to 22 days after conception. You and your care provider can't hear them yet, but it may be possible to see the beating motion on an ultrasound. with these changes, circulation begins, making the circulatory system the first functioning organ system.
This week, not much is happening in the inner layer. It will be awhile yet before those areas take shape. At the moment of conception, your baby was a single-celled zygote and was microscopic in size.
If you haven't been taking a folic acid supplement you should start now.
Although the embryo is only just visible to the naked eye, the spinal column and brain have already begun to grow and a blood vessel has developed which will become the heart. When 5 weeks pregnant the embryo is around 0.25 inch (4 mm) long.