SECOND WEEK The fertilization of the egg takes place about two weeks after the last menstrual period in a regular 28-day cycle. At this stage the pregnancy consists only of the fertilized egg, which is not visible to the naked eye. The fertilized egg travels inside the fallopian tube and enters the cavity of the uterus. It embeds itself into the uterus by the end of the third week. It is still not visible to the naked eye. The fertilized egg that is embedded in the wall of the uterus grows very rapidly and is just visible to the naked eye by the end of this week. High levels of progesterone help change the cells lining the inner layer of the uterus to become highly active and secretory, known as Decidua, which forms a thick layer in the uterus and has a large number of blood vessels. By the end of this week, the baby is about two millimeters in length. Changes include the following:
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