In Aitareya Upanishad, the second chapter talks about how souls are reborn and how life on earth is a product of three births, rather than just one. While it is the mother that bears the child, it is not just her that bears the soul. The father, in his sem[e]n sac, also hosts some part of the soul before it is transferred to the mother.
The chapter also talks about abortion, stating that it is an ethical and moral problem and that people should show more compassion and understanding towards the depth of such practice so that they can get rid of such practice.
The verses also talk about how human birth passes through three phases because there are three souls in play, rather than two, and each birth is a result of past karma. The parent’s karma also has some roles in part of the birth.
The process first starts when the soul departs from this world to the world of ancestors. It stays there till the karmas are exhausted. The bodies of the departed souls in the astral world are formed with past deeds and also the offerings made to them by their descendants.
By the act of God, they gradually exhaust. After doing the time, they fall down to earth through the water in the rain and reach the earth. They enter plants. Some enter animal bodies when the animals eat the plants. And when humans consume these plants and animals, it enters the human body.
Why and how the soul finds the body of its father is hard to explain. But we must understand that karma has a great role to play here. In India, there is a saying, “Who should eat what is already predetermined, and each food grain bears the name of the individual who is expected to eat it.” Thus, Hindus believe that food plays an important role even in the birth and death of beings, not just as an element in survival.
The Vedic beliefs state that it is from food the journey of each individual soul begins in the mortal world, as the soul finds the parent’s bodies through the food. This is a central idea in Vedic theology. Vedas also claim that food is verily of the form of Brahman, that the bodies of beings are formed from food only. The gross parts become part of the gross body (Sthula), and the subtle part becomes part of the subtle body (Sukhsma). This is how Brahman sustains and nourishes the world through food alone.
Food is an important part of Hindu dharma too. No ritual can be complete without the offering of food. The purpose lies in nourishing gods, humans, ancestors, and other beings. Food symbolizes materiality and energy at the same time. The ritual is the transformation and transference of it. The same can be said about the ritual of life. When the individual enters this world, food is offered to the body in a symbolic ritual, and when the individual passes, the body is offered as food to the god of fire.
As mentioned before, the food contains souls, and that enters the male bodies through food and then enters the sem*n sac. It doesn’t happen right away. It is a process and occurs in stages. When the father digests the food, it becomes part of the strength of the father, and as time passes, it gains more vigor and becomes the sem*n (Retas), where it awaits its form into a human body. This is the first birth – the birth in the body of the father.
Through sexual reunion, this soul finds its way into the womb of a woman and thus the soul also becomes a part of the mother’s body. Then, with nine months in her womb, the soul rests. This is the birth in the body of the mother.
As the child emerges out of the womb, it attains its own self and its own body. This is the third birth, a birth in its own body.
This process is quite different from what we normally understand and this is how Vedas explain it. There is a transcendental process in the background of a biological process.
Modern science says that conception happens when an egg is fertilized by a sperm cell. But the Upanishads claim that it happens much before when the soul finds its way from the ancestor world to the earth and becomes a part of the food. It means that birth is not just because of sexual activity. In other words, it means that conception can never happen unless a soul is willing to participate.
This is the exact reason why Vedas prohibit abortion and consider it rather as a mortal sin. Even early-stage abortions are considered to be sins. The Vedas say that when that happens, the soul can fall into the wrong womb and can affect its destiny too.
Thus, the rebirth of a soul is a long, continuous process. Every soul has to find its own father that is worthy enough to hold it. And it stays there for a long time before it reaches its mother fit enough to hold it. Then it develops on its own. And that is a long process.
Anything can happen in between, thus, impeding the rebirth of the soul. Hence, it cannot be said that every soul that falls from the ancestral world will find its compatible father, and then its mother, and then will succeed in its process of rebirth.
That is why Vedas claim that moksha is the ideal way from these ordeals of rebirth.