Each reincarnation depends on so many things. And if a Soul has finally reached the end of their karmic path it may not choose to reincarnate at all. It may choose to remain in pure cosmic consciousness or pure positive energy. Or it may be completely karmically clean and decide to reincarnate anyway to help others on their journeys. These are known as bodhisattvas.
Dr. Helen Wambach, Ph.D., was one of the earliest scientific researchers into past lives and reincarnation. She was the author of Reliving Past Lives and Life Before Life.
In Life Before Life Dr. Wambach described the results of hypnotizing 750 people and taking them to the time between their past and current lives. One of her findings was that the disembodied consciousness, or soul does, not enter the body until near birth. "The soul usually enters the body near birth, and has a choice of which fetus to enter. If one fetus is aborted, it is possible to choose another. In some cases, the soul who will occupy the fetus, is in contact with the soul of the mother, and can influence her decision regarding abortion."
Dr. Wambach found that 89% of those hypnotized said they did not become part of the fetus until after six months of gestation. A large group said they did not join the fetus, or experience inside it, until just before or during the birth process. They existed fully conscious as an entity apart from the fetus and even after six months many reported being 'in' and 'out' of the fetal body.
"Many subjects reported that the onrush of physical sensations on emerging from the birth canal was disturbing and unpleasant. The soul exists in a quite different environment in the between-life state. The physical senses bring so much vivid input that the soul feels almost 'drowned' in light, cold air, sounds.She reported that the new-born baby feels cut off, diminished, alone compared to the between-life state. To be alive in a body is to be alone and unconnected. Perhaps we are alive to learn to break through the screen of the senses, to experience while in a body the transcendent self we truly are."