At what stage is the embryo implanted in the uterus of the mother in the case of test tube baby?

Explanation

**Step-by-Step Solution:**

1. **Understanding Test Tube Baby Concept**: The term "test tube baby" refers to a baby conceived through in vitro fertilization (IVF), where fertilization occurs outside the female body.

2. **Process of IVF**: In IVF, the egg is retrieved from the female (either from the mother or a donor) and sperm is collected from the male (either from the father or a donor).

3. **Fertilization in Controlled Conditions**: The egg and sperm are combined in a laboratory setting, mimicking the conditions of the female reproductive system to facilitate fertilization.

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