At what point in development does an embryo become a fetus?

Options

A. by the twenty-eighth week
B. by the eighth week
C. by the thirty-ninth week
D. by the nineteenth week

Answer

**Step-by-Step Solution:**

1. **Fertilization**: The process begins with the fusion of male and female gametes, resulting in a diploid zygote.
- **Hint**: Remember that fertilization is the first step in human development.

2. **Cleavage**: After fertilization, the zygote undergoes rapid mitotic divisions known as cleavage, leading to the formation of a solid ball of cells called the morula.
- **Hint**: Think of cleavage as the initial cell division that occurs after fertilization.

3. **Morula Formation**: The morula consists of approximately 8 blastomeres and is a solid structure.
- **Hint**: The morula is an early stage before the formation of a hollow structure.

4. **Blastula Formation**: Further cleavage leads to the formation of the blastula, specifically known as the blastocyst in humans.
- **Hint**: The transition from morula to blastocyst marks a significant change in structure.

5. **Implantation**: The blastocyst implants into the uterine wall approximately 7 days after fertilization.
- **Hint**: Implantation is crucial for the embryo to receive nutrients and support from the mother.

6. **Gastrulation**: Following implantation, the process of gastrulation occurs, which is the formation of the three germ layers that will develop into different tissues and organs.
- **Hint**: Gastrulation is a key developmental phase that sets up the body plan.

7. **Embryo Development**: The developing structure is referred to as an embryo until it reaches about 8 weeks of gestation.
- **Hint**: Keep track of the timeline; the embryo stage lasts until about 8 weeks.

8. **Fetus Formation**: At around 8 weeks, the embryo is officially termed a fetus, marking a significant milestone in development.
- **Hint**: The transition from embryo to fetus is a critical point in prenatal development.

9. **Duration of Fetal Development**: The fetal stage lasts for approximately 9 months until birth.
- **Hint**: Remember that the fetal stage is the longest phase of prenatal development.

**Final Answer**: An embryo becomes a fetus at approximately 8 weeks of gestation.

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