The Buddha asked the Brahman Student, Assalayana:
“‘Do you know how there is the descent of an embryo?’
“‘Yes, master, we know how there is the descent of an embryo. There is the case where:
- The mother & father have come together
- The mother is fertile, and
- A gandhabba [the being about to be reborn] is standing present.
The coming together of these three is the descent of the embryo.‘
Source: The Assalayana Sutta: With Assalayana
The means that the spirit of a living being to be reborn in the womb – needs to be around when the mother and father come together (granted the mother is fertile). So this is when life begins – when all these 3 things intersect at the moment of conception!
So once you die, your 8th consciousness leaves your body last and becomes the intermediate skandha body (or what you would call a spirit or ghost – the body between skandhas). Theravadans call this the Pati-sandhi vinnana – the rebirth linking consciousness. It usually stays around for 49 days on average awaiting new rebirth.
The Buddha explains this further in the Mahatanhasankhaya Sutta:
The Birth & Growth of a Being
“Monks, the descent of the embryo occurs with the union of 3 things.
- There is the case where there is no union of the mother & father, the mother is not in her season, and a gandhabba  is not present, nor is there a descent of an embryo.
- There is the case where there is a union of the mother & father, and the mother is in her season, but a gandhabba is not present, nor is there a descent of an embryo.
- But when there is:
- A union of the mother & father,
- The mother is in her season, and
- A gandhabba is present
Then with this union of three things the descent of the embryo occurs.
“Then for nine or ten months the mother shelters the embryo in her womb with great anxiety, as a heavy burden. Then, at the end of nine or ten months, she gives birth with great anxiety, as a heavy burden. Then, when the child is born, she feeds it with her own blood — for mother’s milk is called blood in the discipline of the noble ones.
In explaining the 12 Conditioned Links of Dependent Origination in detail, the Buddha spoke of the same descent of the consciousness of the being to be born into the womb of the mother:
“‘From consciousness as a requisite condition comes name-and-form.’ Thus it has been said. And this is the way to understand how from consciousness as a requisite condition comes name-and-form.
If consciousness were not to descend into the mother’s womb, would name-and-form take shape in the womb?”
“If, after descending into the womb, consciousness were to depart, would name-and-form be produced for this world?”
“If the consciousness of the young boy or girl were to be cut off, would name-and-form ripen, grow, and reach maturity?”
“Thus this is a cause, this is a reason, this is an origination, this is a requisite condition for name-and-form, i.e., consciousness.”
Source: The Maha-nidana Sutta: The Great Causes Discourse
Name and form just means the 5 skandhas:
- Nama/Name – means the sensory consciousnesses of the living being.
- Form/Rupa – means the body of the living being.
i.e., the mind and body of the new living being. In this context, name and form means “life”. So if the consciousness of the spirit to be reborn as the new baby didn’t go into the mother’s womb – there would be no new life.
And if the spirit leaves, then there would be no new life. So this probably happens in cases of stillbirths and miscarriages.
Implications of this
So if human life begins at conception, then this has implications for abortion because it would be killing a baby. So if at all possible, you’d want to avoid being put in tough situations like that.