Ultrasound vs. LMP: Understanding Fetal Age Discrepancies

Question
Hi,
My LMP started on Sept. 30 2010, according to that, my baby should be 10 weeks old.  But I just went to the doctor's office and according to the ultrasound size estimation, the baby is 11 weeks old.  I know that all baby in the first trimester develop at the same rate, so I was concerned that this baby is bigger than he/she should be.  I am certain of my LMP because we were trying and I was tracking my period.  I am always pretty regular with a 26-28 day cycle.  I just wonder if this is any other explanation of the age difference and whether there is any cause for concern.  (by the way, the doctor did the ultrasound vaginally as part of my first visit, it wasn't the big ultrasound with a sonographer to check everything)

Answer
Hi Emily,

There are a few reasons it could be off some, but in any case there is no cause for concern.  Some women do not ovulate on the same day of every month, even if they experience regular cycles, so it's possible that you released the egg a little early than expected.  Also, while we strive to be as accurate as possible when measuring the baby, sometimes it doesn't always happen.  This could be the doctor over measuring when placing the calipers from baby's head to bottom.   There is an average of up to 5 days in range of accuracy.  Based on my pregnant wheel calculator it shows today that you are 10 weeks and 2 days along (based on your last period).  Did you have the ultrasound done today Dec. 10th?  Maybe it is a combination of the two scenarios?  A bigger than expected baby in the first trimester is no cause for worry.   Congratulations on your pregnancy!

Take Care,
Sara