What is A1c and is it an accurate reflection of blood sugar in pregnancy?

In diabetes a lot of attention is paid to the A1c. What is it and is it good at reflecting over sugar levels?
Simple average blood sugars are not that great at telling how the blood sugar control is really like.  Consider Mr Smith who has sugars ranging from 2 to 20 mmol/l (36 – 360 mgs/dl) and Mrs Dixon with sugars of 10 to 12 mmol/l (180 – 216 mgs/dl).  Both have the same average of 11 mmol/l (198 mgs/dl) but the control of sugar in Mr Smith sounds much worse.  The A1c is a much more stable reflection of overall sugar control.
Red blood cells survive for 3 months in the circulation.  They are mainly made of hemoglobin and a sub component of hemoglobin is A1c.  Normally some of the sugar in the blood stream sticks to the A1c (this is called glycation) so that at any given time some 4.1 to 6.1% of the A1c is glycated. If the blood sugar is high over the last three months (for the life of the red blood cells) then a greater percentage will be glycated.  There is a pretty strong correlation between the average blood sugar and the percent A1c.  Thus an A1c of 6.5% is fairly good for someone with Type 1 diabetes, 8.0% would indicate things are not great and 10% or higher the sugars are not controlled. In some countries the units for A1c have changed so 6% is now 42, 8% is 64 and 10% is 86.
If something changes the life span of the red blood cell, this can change the A1c even if the glucose is unchanged. Thus a transfusion, or bleeding and the body making new red blood cells will lower the A1c, anemia where the cells hang around longer increases the A1c. In normal late pregnancy the red blood cells stay around longer and the A1c normally rises a little, about 0.4 %. Liver disease and some conditions with abnormal haemoglobin decrease the A1c and kidney disease increases it. Some people naturally have longer lasting red blood cells or the sugar enters the cell more easily and they will a have higher A1c.
Overall though, the A1c is a good reflection of sugar control and should be under 7%  (53 in the new units) at time of conception for  safety (See Blog for Sept 1st, 11).  In our clinic we check it every month during pregnancy to be sure we are keeping things well controlled. It is less useful in gestational diabetes because the rise in sugar is only recent. Occasionally though we can have great sugars but the A1c is higher than expected. Assuming the meter is accurate and there are enough readings to tell what is happening all day long, then maybe why the A1c is off due to the red blood cells are surviving longer in this person or the sugar is going into the cell more easily. Most times the test is a true measure of sugar control and hence the attention paid to it especially in pregnancy when we need good blood sugar control.