Hi i don't really know anything about this so i just wanted to tell you there are questions just like yours on this site that could help you just go to search and put in pregnant but no baby in ultrasound you'll find answers and women to talk to who went through the same thing i hope this helps you
Your doctor may have been off on her estimate and you aren't as far along as they think. Did they ask you when the first day of your last menstrual period was?
hCG levels in pregnancy have such a wide range of normal it is impossible to say how far along you are with any degree of precision based on hCG levels. Your result, 15,000 mIU/ml is is within normal range for between 6 to 8 weeks. That is a pretty big range. If you are 6 weeks instead of 8, it is very possible baby doesn't show up on an ultrasound yet.
Normally, in addition to the 2 blood draws to measure how much your hCG levels are increasing, your doctor will also do a repeat ultrasound in a week or two. Quite often it is just a case of baby not being as far along as they thought and the 2nd ultrasound shows a healthy but younger than expected baby.