The terms embryo and fetus refer to different stages of development in human pregnancy.

Embryo

- Definition: The term "embryo" is used to describe the early stage of development from fertilization until the end of the 8th week of gestation.

- Developmental Stage:

- Weeks 1-2: Fertilization occurs, and the zygote forms, undergoing cell division to become a blastocyst.

- Weeks 3-4: The blastocyst implants in the uterine wall, and basic structures begin to form, including the primary germ layers (ectoderm, mesoderm, endoderm).

- Weeks 5-8: Organogenesis occurs, where the foundations of major organs and systems are established, and the embryo starts to take a more human-like shape.

Fetus

- Definition: The term "fetus" is used from the start of the 9th week of gestation until birth.

- Developmental Stage:

- Weeks 9-12: Continued growth and maturation of organs. The fetus begins to make movements, although these are not yet felt by the mother.

- Weeks 13-24: Rapid growth and further development of systems. The mother can start feeling fetal movements (quickening), and the fetus's external genitalia become distinguishable.

- Weeks 25-40: The fetus continues to grow in size and weight, with final maturation of organs and systems in preparation for birth.

### Summary

- Embryo: From fertilization to the end of the 8th week. Key processes include cell division, implantation, and initial organ formation.

- Fetus: From the beginning of the 9th week until birth. Key processes include growth, organ maturation, and preparation for viability outside the womb.

Understanding the distinction helps in identifying specific developmental milestones and potential medical interventions during pregnancy.