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The Embryonic Period
Week 4 signals the beginning of the embryonic period, which will continue until week 10. The blastocyst becomes embedded in the uterine wall. At this point, the blastocyst has grown into a disk-like structure, and is two layers thick. These two layers, called the epiblast and the hypoblast, divide the blastocyst into two chambers, the amniotic cavity and the yolk sac. Additionally during this period, your baby's organs will begin to develop.