Describe the development of a human from the first steps after fertilization to the end of the 9th month of pregnancy. Include the major developments during each trimester of pregnancy.

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- **First Trimester:**
- **Weeks 1-4:** Fertilization occurs, and the zygote forms. The zygote undergoes rapid cell division and becomes a blastocyst, which implants into the uterine wall.
- **Weeks 5-8:** The embryo forms, and major organs begin to develop. The heart starts beating, and structures such as the brain, spinal cord, and limbs start to take shape.
- **Weeks 9-12:** The embryo transitions into a fetus. Major organs and systems continue to develop, and the fetus starts to make small movements.

- **Second Trimester:**
- **Weeks 13-16:** The fetus grows rapidly, and its body becomes more proportionate. Facial features develop, and the fetus starts to make sucking motions.
- **Weeks 17-20:** The mother may begin to feel fetal movements. The fetus develops a fine layer of hair called lanugo and begins to produce vernix caseosa, a protective coating on its skin.
- **Weeks 21-24:** The fetus's organs continue to mature. It starts to respond to external stimuli, such as sounds. The lungs develop but are not yet fully functional.

- **Third Trimester:**
- **Weeks 25-28:** The fetus continues to grow and gains more weight. The nervous system and brain continue to develop, and the lungs start producing surfactant, which is crucial for breathing after birth.
- **Weeks 29-32:** The fetus's bones fully develop, although they remain soft. The eyes can open and close, and the fetus continues to accumulate body fat.
- **Weeks 33-36:** The fetus's movements may become more restricted due to limited space. The immune system strengthens, and the lungs and other vital organs continue to mature.
- **Weeks 37-40:** The fetus is considered full-term. It prepares for birth by moving into a head-down position. The body continues to gain fat, which helps regulate body temperature after birth.