Low hcg doubling every 64 hours

Hannah • 5 beautiful children 14 angel babies pregnant and shooting for #6: the last one

Can anyone provide me some comfort? I am 4 weeks 3 days today. We caught the pregnancy as soon as I implanted! Hcgs went:

1/29 3

1/31 11.5

2/4 31.5

According to the chart my hcgs are doubling every 2.5 days. Which I keep reading between 48-72 hours is normal. I'm just worried because my numbers are low. At 4 weeks I should at least be 50 right?? I have had 8 miscarriages with the most recent MC in September. I just want some comfort.