My claim is based on my personal experience of seeing what women have to go through and risk to bring a child into this world, and total fertility rate dropping to less than slightly above 2 at best in every single country where women achieve education and civil rights, indicating they obtain the ability to say no to sex in situations where they can get pregnant, and/or they obtain the ability to prevent or abort pregnancies.
https://ourworldindata.org/fertility-rate
All the countries with high total fertility rates have environments where women would not expect quality healthcare or medical interventions should things go wrong. It stands to reason that women are not making a conscious decision to put their quality of life and life itself at risk over and over again to birth human resources.
Outside of outlier cult-ish groups, I see almost no women choosing to have more than 3 children. And the data seems to agree. Everyone I know that had a 3rd child, they had 2 boys or 2 girls first and only had a 3rd to try for the other gender.
So I am deducing that the painful experience of pregnancy/childbirth/breastfeeding/infant rearing is a significant limiting factor.