The fertilized egg cell reaches through the oviduct to the uterus about 4 days after fertilization. At the end of the week, the fertilized egg cell or zygote is implanted to the uterine lining and the conception period ends. You’re pregnant! By that time the fertilized egg cell has divided and formed a cluster, called the embryo. The outer cells of the embryo have the ability to dig deeper into the uterine lining in order to connect with mother’s blood vessels. Inner cells, however, are divided into three germ layers, the endoderm, the ectoderm and the mesoderm. The endoderm develops to the digestive system and respiratory system, and organs associated with the digestive system, such as the liver and pancreas. The mesoderm is found between the ectoderm and the endoderm and develops to somites, which form muscle; the cartilage of the ribs and vertebrae; the dermis, the notochord, blood and blood vessels, bone, and connective tissue. The ectoderm develops to epidermis, and to the neural crest and other tissues that will later form the nervous system, hair, skin and eyes.
If twins are born, the division takes place before the fifth day after fertilization in 1/3 of the cases.
You don not have a clue, taht you are pregnant. Some women, however, feel nausea and breast tenderness. When a fertilized egg cell attaches to the uterine lining, there may be some bleeding.