I would wonder with
your levels though, if you look up levels by week
, 5 weeks is said to be 18-7,340, and 6 is 1,080-56,500. I found out with my daughter at 3 weeks 4 days too. I don't think that has anything to do with the number of babies, just implantation. After ovulation it takes 6-12 days for implantation, and then another 24-48 hours for the levels to be high enough for a positive test.