At what point does hcg stop doubling every 48-72 hrs?

My doctor had me go for bloodwork Tuesday and Thursday to check hcg levels due to some spotting. I’m waiting for the doctor to call me back but my results were

Tuesday: 37603

Thursday: 50875

They obviously didn’t double and now I’m kind of nervous. The spotting as slowed down and I’m not having any cramping. I’ll be 7 weeks tomorrow. Is this a normal increase? Or should of they doubled?