How do I modify traits to make them hierarchical?

[BFDCYY]

I want to modify the "pregnancy" trait, making it a trait with levels and experience like "hunter", and the trait experience will increase over time, with each level increasing or decreasing some values (I haven't thought about it yet). After ten months of pregnancy, the "pregnancy" trait will reach the maximum level, the child will be born, and the trait will disappear.