When does hcg slow down ?

I read that hcg slows down after it hits 6,000 or it doubles every 3 days instead of 2 days is this true ? My last beta hcg they were doubling every 48 hours and the last one was 6,300 on the 29th and then due to some bad cramping my doctor ordered it on the 5th so about 6 days later it was 27,800 is this normal or should it be higher ?