Why is my dr. concerned that I might have a miscarriage?

At 4 wks. my first hcg level was 134, 2 days later it was 222, and 4 days later it was 901. I had a sonogram today at 5 wks. and dr. saw the yolk sack which was implanted in my uterus. The age of the embryo was 5 wks, 1 day. Said my levels were low in 4th wk that's why he's concerned about miscarriage. I don't understand why. Can anyone help?