**Step-by-Step Solution:**
1. **Understanding Gametes**: In mammals, reproduction begins with the fusion of two gametes: the male gamete (sperm) and the female gamete (ovum).
2. **Formation of Zygote**: When the sperm fertilizes the ovum, they form a single cell called a zygote. This is the very first stage of embryonic development.
3. **Development Stages**: The zygote undergoes several stages of development. Initially, it divides and forms a structure known as a blastula, which eventually develops into a blastocyst.
4. **Implantation**: The blastocyst travels down the fallopian tube and reaches the uterus. It then implants itself into the uterine wall. This is a crucial step for the continuation of development.
5. **Embryo Growth**: After the blastocyst implants into the uterine wall, it begins to develop into an embryo. This is when the actual growth of the embryo starts.
6. **Conclusion**: Therefore, the correct answer to the question is that in mammals, the embryo begins to grow soon after implantation.
**Final Answer**: Option A: Soon after implantation.
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