How does someone know if there amino fluid is going down? And what week do women normally start prenatal classes?

My mother keeps telling me I should start in prenatal classes but i’am only 28 weeks pregnant. I was thinking of starting when i’am 32 weeks. But what do you guys thinks it’s best to start? Also I was reading stories on some people saying each visit they had at the doctors they kept loosing their amino fluid. What’s the cause of loosing the fluid or having very little fluid besides when it’s close to giving birth

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