4 Weeks Pregnant
The fourth week of pregnancy marks the beginning of the embryonic period, which is when the blastocyst begins to transform into an embryo. The embryonic period is when the baby's brain, spinal cord, heart, and other organs begin to form. Your baby is now about 1/25 of an inch long (0.04 inches).
The embryo is made of three layers:
- The top layer, the ectoderm, will give rise to a groove along the midline of your baby's body. This will become the neural tube, where your baby's brain, spinal cord, spinal nerves, and backbone will develop.
- Your baby's heart and a primitive circulatory system will form in the middle layer of cells, which is called the mesoderm. This layer of cells will also serve as the foundation for your baby's bones, muscles, kidneys, and much of the reproductive system.
- The inner layer of cells, the endoderm, will become a simple tube lined with mucous membranes. Your baby's lungs, intestines, and bladder will develop here.
The placenta has also started to form -- it is producing hCG, among other hormones, which is an important hormone in pregnancy.
This is a very emotional time for the mother, and she may be experiencing early symptoms of pregnancy such as mood swings and nausea. Just remember that these are completely normal symptoms and that they are directly related to the pregnancy.