Could it be twins?
Q: I am 18 DPO and my serum hCG is 556. Could this mean I have twins?
A: A serum hCG of 556 mIU/mL at 18 DPO is normal both for one or two fetuses. You could have twins, but you can't really tell from the serum hCG value alone. At 18 DPO the serum hCG for singletons is normally between about 70 and 750 mIU/mL, and for twins it can be between 200 and about 1750 mIU/ml. More hCG Facts HERE.