HCG levels rising but not doubling?

Nancy • SAHM to 4, miscarriage 1, pregnant #5 with a girl 💜Due 5/15/19, C-Section scheduled 5/10/19

So I had my levels taken this past Friday morning and it was at 13,072 and I went in today in the morning and they are now 18,810. They didn’t exactly doubled. My case worker ( I was assigned one since I had a miscarriage last month) said she isn’t too concern but they should be doubling. I need to go back Thursday morning again to get my levels checked. Plus I have an ultrasound this Friday. I am a little nervous because they aren’t doubling like they should! This is my first time getting my levels checked. None of my other pregnancies were checked. Any one has been through a similar situation?