The truth is there is no answer, no one knows [when the "fetus" receives its soul]
No one knows when the soul comes into the body.
According to Jeremiah 1:5, babies have souls before they are born. They existed before they were born. "Before I formed you in the womb I knew you..."
God also knows them and has plans for them.
11 For I know the plans I have for you," declares the LORD, "plans to prosper you and not to harm you, plans to give you hope and a future. Jeremiah 29:11
The Vedas state that the soul enters with via the sperm, so when it unites with the egg, the soul's energy causes growth. That without the soul, there is no growth.