Enlarged Yolk Sac and Fetal Heartbeat: Understanding Early Pregnancy

Question
My last menstrual period was the end of December 2007. Around Jan. 16,2008 I just knew I was pregnant so I took a pregnancy test. It was positive. I went to my family doctor to confirm this and I was right.  My doctor cannot tell me when I am due everytime we go to the doctor he says I don't think your as far along as we thought. I had an ultrasound 3 times now. The last one showed an enlarged yolk sac I don't know what size he didn't tell me but we also saw a heartbeat on ultrasound. I am a diabetic and 34 years old and I have to sons now. What can be wrong?

Answer
Dear Tammy

If you had the first day of your last period on the end of December and if its length is 28 then you should be pregnant around 10 weeks. however, if you had 3 ultrasound since the end of december, your doctor should be more capable than me in telling you the exact duration of your pregnancy and your due date.