Title: Fetal development: MedlinePlus Medical Encyclopedia
Headings: Fetal development
Fetal development
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Content: Week 4
Once the blastocyst reaches the uterus, it buries itself in the uterine wall. At this point in the mother's menstrual cycle, the lining of the uterus is thick with blood and ready to support a baby. The blastocyst sticks tightly to the wall of the uterus and receives nourishment from the mother's blood. Watch this video about: Cell division
Week 5
Week 5 is the start of the "embryonic period." This is when all the baby's major systems and structures develop. The embryo's cells multiply and start to take on specific functions. This is called differentiation. Blood cells, kidney cells, and nerve cells all develop. The embryo grows rapidly, and the baby's external features begin to form.