can anyone help or shed some light?

Laura
My auntie has found out she is pregnant after 7 years of trying with one miscarraige. She had a blood test that indicated her levels were very high and  she was further along than what she thought, 2nd blood test (two day later) only showed a 15% increase In HCG levels... which the doctor wanted to double. 
She's had a scan and it's shown a sac but not visible baby?? Which is strange considering her high levels of HCG which indicated she was further into her pregnancy than what she thought. Her last period was on 28th October 
Any help or idea of what could be going on would be great thanks