The embryonic stage is characterized by rapid cell division and the formation of the major organs and systems of the body. By the end of the embryonic period, the fetus is fully formed and looks like a miniature human being.
The process of embryo development into a fetus can be divided into three main stages:
1. The pre-embryonic stage (weeks 1-2): This stage begins with fertilization, when a sperm fertilizes an egg. The fertilized egg, called a zygote, begins to divide rapidly. By the end of the second week, the zygote has formed a blastocyst, a hollow ball of cells that implants into the wall of the uterus.
2. The embryonic stage (weeks 3-8): During the embryonic stage, the major organs and systems of the body begin to form. By the end of the eighth week, the embryo is fully formed and looks like a miniature human being.
3. The fetal stage (weeks 9-40): The fetal stage is characterized by the growth and development of the fetus. By the end of this period, the fetus is ready to be born.
Here are some of the key events that occur during embryo development into a fetus:
* Week 3: The heart begins to beat and the neural tube forms.
* Week 4: The arms and legs begin to develop.
* Week 5: The eyes, ears, and nose begin to form.
* Week 6: The fingers and toes begin to form.
* Week 7: The sex organs begin to develop.
* Week 8: The fetus is fully formed and looks like a miniature human being.
By the end of the first trimester (week 12), the fetus is about 3 inches long and weighs about 1 ounce. Over the next 28 weeks, the fetus will grow rapidly and mature. By the end of the third trimester (week 40), the fetus will be about 20 inches long and weigh about 7 pounds.
