Most of the organ systems are formed

Options

A. At the beginning of first trimester
B. By the end of first trimester
C. At the end of third trimester only
D. Only at the end of second trimester

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks when most of the organ systems are formed during human development after fertilization.

2. **Fertilization and Zygote Formation**: After fertilization, the zygote is formed. This zygote undergoes several divisions and eventually develops into a blastocyst.

3. **Implantation**: The blastocyst attaches itself to the endometrium (the uterine wall). This is crucial for the establishment of pregnancy and the development of the placenta.

4. **First Trimester Duration**: The first trimester lasts about 12 weeks. This is a critical period for organ development.

5. **Development of Organ Systems**: By the end of the first trimester, most of the major organ systems are formed. This includes the development of limbs and genital organs.

6. **Second and Third Trimesters**:
- The second trimester lasts until about 24 weeks, during which fine hairs, eyelashes, and eyelids develop.
- The third trimester lasts from 24 to 40 weeks, leading to the formation of a fully mature baby.

7. **Conclusion**: Therefore, the correct answer to the question is that most of the organ systems are formed by the end of the first trimester.

**Final Answer**: Most of the organ systems are formed by the end of the first trimester.

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