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3 st week of pregnancy: the formation of the umbilical cord
The embryo is at this stage composed of three parts: the ectoderm, the mesoderm and the endoderm. They differentiate to form the skin, central and peripheral nervous systems, eyes, ears, bones, ligaments, kidneys, part of the reproductive system and your baby's lungs and intestines respectively. The umbilical cord is now visible and connects the embryo to the mother through the placenta, called the chorion at this gestational age, which is beginning to set up.