Calculating Gestational Age: A Pregnancy Timeline

Question
I am 27 with my first pregnancy.  My LMP was 29 Nov 2005. Which I am pretty sure of.  We were trying to conceive I tracked my periods.  I had come off Depo in June 2005. Had my first period in Oct 2005. Which was very irregular and long before my 29 Nov 2005 cycle. I was being followed by my GP for the first few months where everything has been fine and I have been measuring according to my LMP dates.  My last check was when I was 16 weeks. I just saw my OB/GYN for the first time this week.  However, instead of measuring 22/23 weeks I have jumped all the way to 28 weeks.  My first ultrasound is not set for the 16th.  With this size difference I am a bit nervous to see the outcome of the ultrasound so I know what to expect - change in delivery date; twins; large baby.  I had quick weight gain right fromt he start but have not had any significant amounts of the usual pregnancy complaints - most of all no morning sickness.  I was wondering if at 22 weeks I would already be seeing physical signs of fetal movement?  I can see obvious signs on my abdomen when the baby is kicking / punching ect. & can my husband can easily feel the baby move around. Or could this be a sign that supports an older gestational age?  My family has no history of twins but my husband's grandmother is a twin. Just wondering if you can shed some light into what we might expect at the ultrasound.  What differences would we expect to see if the baby was 22 weeks versus 28 - or by that time 24 verus 30.
Thanks.

Answer
The best time to perform a sonogram is at 20 weeks gestation (from the first day of your last period. If your last period was November 29 and it was a normal period, you should now be 22-23 weeks gestation. In a first pregnancy, we usually start to feel kicking and fetal movement at 22 weeks, so it appoears correct. if the obstetrician thought you were large for dates, a sonogram will reveal what is going on. Don't worry.Either you are carrying high, have more than one bsby, have a large baby, etc. The sonogram will also confirm your due date. Just be patient..It would be better if you could take the sonogram earlier than the 16th because you would then be 23-24 weeks gestatioinal age. Any sonogram after 21 weeks is not as accurate as it gets laater and later.