Didn't really answer my question

  One of the questions in my last post was whether an HCG from 331 to 656 over three
  days was "close enough" to doubling to be considered normal.  I'm not sure how
  exact the rule is.  THanks.  
Dear Lorraine:
The values were measured in two laboratories: this alone can explain a difference between 656 and 331. The range of normal would be approximately 550 to 771.
Keywords: hCG doubling
This information is provided for education purposes and is not a medical consultation. If you have specific questions, please speak with your physician.