When does the soul enter the body?

That is a complicated question. I suppose it would be helpful to first define the word "soul", (as opposed to "spirit", which is not necessarily identical). According to the Bible, all living creatures have a "soul", but only mankind has the capacity to commune with God, (soul/spirit).

In Genesis 2:7 it reads, "...and man became a living soul". "Living soul" in that verse is translated from the same Hebrew words used in Genesis 1:24 for "living creature", (in reference to animals). The Biblical definition of soul refers simply to "life", (or the animating force), which originated from God, the Source of Life. Therefore, I would say that the soul "life" begins at the very moment the united egg and sperm begins to develop and grow within the womb.

The difference in the soul of man from the animal kingdom is that God created man in His own image, (a spiritual resemblance to our Creator). In addition to being a living creature, man also possesses a moral and rational conscience, intellect surpassing animals, a capacity for eternal life/fellowship with God, self-consciousness, speech and worship.

In the account of God's creation of Adam, in Genesis 2:7 the scriptures read, "And the Lord God formed man of the dust of the ground (physical body), and breathed into his nostrils the breath of life (spirit)"... Life in this context is derived from the Hebrew word "ruach" which refers to "spirit"...."and man became a living soul."

Adam's physical body, (aside from the "life" from God) did not possess his "soul". The soul of man is the part of us that is "alive". The word "soul" in reference to mankind, speaks of the whole person. When we die, the "soul/spirit", (life force), departs from the body. Therefore I would conclude that the soul/spirit of man simultaneously is created in the living fertilized egg that is just beginning to grow and develop inside his/her mother's womb. If the life force, (soul) was not in the fertilized egg, it would simply be "dead" physical matter that could not be "alive", (like the body of Adam before God gave him life).

As mentioned by Goldengirl, it is mentioned in the scriptures that John the Baptist was filled with the Holy Spirit from inside his mother's womb. The psalmist in Psalm 139 declared how God knew him at the earliest stages of his development in his mother's womb, which again confirms that the "life/soul/spirit" began from conception when the embryo became "alive".

Psalm 139:14-16 reads, "I will praise Thee; for I am fearfully and wonderfully made: marvelous are Thy works; and my soul knoweth right well. My substance was not hid from Thee when I was made in secret, and curiously wrought in the lower parts of the earth. Thine eyes did see my substance, yet being unperfect, and in Thy book all my members were fashioned, when as yet there were none of them. How precious also are Thy thoughts toward me, O God! How great is the sum of them."