Understanding Yolk Sac Size in Early Pregnancy: A Doctor's Explanation

Question
It has been 9 days since my last sonogram, and the sono is showing 9 days worth of growth based on crown rump length, showing that I am just now 8 weeks pregnant.  This information still does not correspond to my lmp, which would put me at 9 1/2 weeks.  My main concern is the size of the yolk sac.  It measured 8 mm last week and 7mm yesterday.  Although he was more positive, he said I am not out of the woods yet, and wants to do a follow up in two more weeks.  What does this mean, and what is he not telling me?  What are my chances of having a HEALTHY baby.  Please be honest.    -------------------------
Followup To
Question -
age-31
2 children, no previous miscarriage
lmp January 15, 2006

Based on my lmp and conception date, I am 8 weeks pregnant.  I am 99% sure of both of these.  I had a vaginal ultrasound yesterday and did hear a heartbeat. However,  the crown rump length and yolk sac only measure 6 weeks 5 days.  Because of my certainty of lmp and conception, my doctor is concerned and wants to do another sono in 10 days.  Why is he concerned?  Is there a chance I will miscarry?  If I don't miscarry, will the baby have problems?
Answer -
If your last period started on January 15, you should now be 8 weeks 4 days pregnant today. A sonogram at this time is not very accurate in determining gestational age. A sonogram is good to determine if there is a fetus and a fetal heartbeat. The best time to take a sonogram is at 12 weeks (from your last period) or 20 weeks (from your last period). If the next sonogram shows adequate growth of the fetus, you need not worry.The baby should not have any problems. If there is no further growth seen on the next sonogram, it may indicate a non-viable fetus. If the growth is adequate and you do see a fetal heartbeat, you should not worry. The risk of miscarriage is 15-20% of every pregnancy. Be patient and take a sonogram again in a few weeks.

Answer
The size of the yolk sac decreases as the fetus grows. If there has been adequate growth in 9 days (9 days of growth) there is a good chance that all will be fine and you may merely have conceived later than you thought. I would wait another two weeks and take another sonogram. It should show the fetus to be 11 weeks if growth was adequate. Then repeat the sonogram at 20 weeks to accurately determine your due date.