self-fulfilling prophecy/placebo effect made manifest
Once long ago few children could be salvaged by cesarean section. New methods and spells for healing during the process were found and used and the first child was born by it. This particular child, through sheer luck, had no mana. There's no saying why, or maybe there is that's up to you. After that event, which could be given a name, it was "known" that cesarean section-born children had low or no mana.
What nobody has caught onto, though, is that the amount you have depends almost entirely on how much those around you in your first few moments believe you will have. The conscious and subconscious thoughts adjacent to the newborn, the placebo effect made manifest by mana and will stick to a child. The gateway for mana to enter must be shunted open at the start, or risk it being forever closed and kick-start the engine of magic within us must be stoked by fires present at birth. A child born into a room full of doctors with the highest hopes will inundate the child's first moments with magic.
This could encourage large families, or births being a culturally celebrated thing. Over the course of countless generations the largest families with highest hopes for their children will have the most successful children. Those children will carry that tendency to have large families on with them, and the culture would adjust such that the largest a family you have the better. Jargon could be invented to this degree likening large families to wishing good luck. Murder would be, in a way, even more tragic. Forcefully shrinking someone's family will leave their next children a little bit less capable. Given enough time this could see the complete extinction of a lineage.
You could have someone break the wheel, a child born by cesarean but with strong mana because the people near them at birth were delusional or just ignorant of an otherwise very prominent stigma. This gives you an out so that this rule can be broken if it serves the narrative.
The more individuals near the newborn that think it will have mana, the more it will. It's a self-fulfilling prophecy.