As I understand it, the soul begins its descent about a month before
the end of gestation. Assuming everything is normal, I guess we'd say that
the actual "physical" descent of the Soul into the body and Matter Aspect
occurs at about the eighth month.
Here's the basic process, as I understand it.
Energy is created, and energy creates matter. The energy that is created (at conception) tends to determine the vibration - and this puts out a "magnet" - corresponding Souls with a similar or higher "vibration" as that put out by the mated couple are attracted to that "possibility" of incarnation. Several factors are then reviewed (yes, there is, or was, a Review Board) and they basically go through the candidates, and a selection is made based on karma, opportunity, lessons to be learned - both by the incarnating Soul, the parents, and the karmically-connected associations of the family and environment.)
There are other factors, and sometimes the process is almost in reverse - I "found" my current family because I had karma I needed to work out - and they with me - so this was an opportunity my parents and I actually worked out LONG in advance. That happens, but is rare, I think. In NORMAL cases, it is the law of Mutual Attraction, on a case-by-case basis.
At about the 4th month, when it starts looking like the baby is really going to happen, a Soul is selected, and that Soul begins "training" on the Other Side - he/she is "introduced" to the parents, facts are exchanged, the Soul begins to become acquainted with the expecting couple or mother. This is a time of acclimation for all parties directly involved.
At about the 7th month, the Soul consciousness begins to "fade" - the persona (or personality) begins to lose its distinct edge and traits, and becomes a little more "universal". The Souls begin to lose their memories of their last lifetime, and they begin to disconnect with the "physical energy/karma" associated with that persona.
In the 8th month, they begin to "drop" - or Descend into the Physical Plane.
The energy of attraction is created at conception, but the persona Soul does not begin to integrate until about the 8th month. After that, there is a magnetic integration/filter system that works naturally between the mother and child.
Some say that babies are fully integrated Souls, and they "lose this" in early childhood, partially because of physical (plane) compulsion, and partially because some parents tend to repress their children's natural expression. It can be recovered (Soular awareness) but it has to be done consciously, through Love, and Will. The more Love (that) is expressed toward the child, the more likely that child will retain his purity of essence - or be able to recover it more easily later in life.
And then, there are the born "demons from hell" - again, Law of Attraction, and also karma - but not necessarily for the parent. Good parents with rotten kids are not always karmically responsible - in fact, they rarely are. Sometimes it is the parents who are giving the Soul another chance to work out his/her karma, but that Soul fails to do so, or chooses not to.
The Soul does not fully "seat" the persona until approximately the 20-30th month after birth (hence, the "terrible twos" when the rebelling persona tries to overthrow the Soul like a unbroken bronc...this doesn't happen that way with everyone.) Before that time, the Soul is given the option to withdraw and let the newly formed persona "die" without engendering karma in the process (sort of a spiritual "add/drop" period); hence the inexplicable nature of SIDS (Sudden Infant Death Syndrome.)
There are some who believe that ALL Souls will have to be re-born, before Ascension, to help them through a final initiation of some sort. I am NOT one of the people who believes this; I believe that all Humans can make choices from either side of the Veil - and that all who CHOOSE to walk through the Veil can do so from either side.
The choice to be dense or non-dense is almost moot; all Humans are effective, as long as they exercise Purpose, Power and Will. It's not that hard to do. It is simply a matter of intent. That "intent" will carry Humans where they need to go - no need to worry about initiations, purgatories, or anything else.
Souls go where they want to go - and are attracted to the vibrations - and Lessons - that suit them.