In the US, it is illegal to use race (or gender), both of which are highly predictive of health care costs. You can use age and smoking, but that’s about it, at least on individual issuance under ACA. The age increase is about 50% of what the true actuarial cost is, so the young subsidize the old.

The result is a healthy 26mo white man gets overcharged and a pregnant, unhealthy, overweight 40yo black woman is undercharged. Usually this just means the 26yo chooses not to get insurance.

(Men are cheaper because they don’t have as many routine medical exams, they don’t go in to the doctor generally even when they should, they don’t get pregnant and their health problems seem to include sudden out-of-hospital death more than chronic conditions. That makes them substantially cheaper to insure even if not actually “healthier”.)

Pre existing conditions and known upcoming things (like, planning to get pregnant) or superior knowledge of one’s health status and risk profile make health insurance a really defective marketplace.