Most women have watched someone go through a difficult pregnancy in their lives before they hit 20. It's not a "belief" for the people who live through it.

People used to die – frequently – from this "belief." And women still do.

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Someone called this a "belief."

There's a 10% to 4% probability that the average teenage girl will die from childbirth, given the cumulative risk of pregnancies in nations without modern medicine.

That's the default state of the human condition. Maternal mortality is frequent and 1 in 25 to 1 in 10 for women without modern medical interventions.

see: The probability that a 15 year old girl eventually dies from a pregnancy-related cause, assuming constant levels of maternal mortality and number of births per woman. https://ourworldindata.org/grapher/lifetime-risk-of-maternal...

There's a reason why the more women become aware of the risks and downsides of pregnancy – the less likely they are to go through with it. Even when indoctrinated from the start. The only sane solution in an otherwise insane world is technological, external gestation / exowombs.