Only a ray of the soul enters the body:[1]
While every Jewish body contains a G-dly soul, the entire G-dly soul does not descend and become invested within the body but rather a mere ray and glimmer. A ray and glimmer of the G-dly soul descends below into the body through contractions. However, the entirety of the G-dly soul and its source and root remains above in Heaven and is called a Mazal. On this it states that man contains a Mazal and that everything is dependent on the Mazal [as will be explained next that a healthy G-dly soul contains a constant communication with his Mazal].
[1] Likkutei Torah p. 71b