Week 4 During Week 4 of pregnancy, the single cell zygote divides many times to form a ball of cell...
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Week 4 During Week 4 of pregnancy, the single cell zygote divides many times to form a ball of cells, with an inner group and an outer shell, known as a blastocyst. The blastocyst reaches the uterus and within a couple of days, implants itself in the wall.

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