When does the clock start ticking- just at what time should I check my blood sugars if I have gestational diabetes?

Gestational diabetes is typically a milder diabetes so does not have much in the way of symptoms or complaints.  Most of what people feel that may be symptoms of diabetes, going to the bathroom frequently, tiredness, feeling warm or hungry relate much more to the pregnancy itself and not the diabetes.  Because of this we have to check the blood sugar as how one feels does not give an accurate sense of how high the blood sugar may be.  Some people may feel their sugar is high or low but it does not answer how high or how low.  So we need real numbers as it is on these absolute values a decision on whether more treatment needs to be started is based.  However, a question we face in the clinic is to decide when should one test the blood sugar and if we do suggest one or two hours after a meal just when does the clock start ticking?
Nearly always we need to know the glucose or sugar level before breakfast.  It tells us are we starting the day on the right track and as explained previously (See Blog Morning Sugar Rise 8th May 2011) represents how well the pancreas is controlling the liver which is always trying to make glucose.  If the fasting blood sugar is up then some treatment if often needed as it is the least responsive glucose value to diet or nutrition changes. The only exception would be if someone is taking a huge snack or meal at bedtime or consuming a snack during the night.
Most times in gestational diabetes the sugar goes too high after a meal and then gradually settles. Thus pre lunch, pre supper or bedtime values are often okay but directly after a meal the numbers are high.  In women who do not have diabetes the peak sugar occurs just a few minutes after an hour, a bit longer if one is over weight.  We have standard targets set for one and two hours after meals so usually use these time points (See targets under Gestational Diabetes Management).  Some clinics focus on the one hour, others on the two hour time points – both work well.  In our clinic we look at what happened on the oral glucose tolerance test when the woman was diagnosed – if the one hour level was more abnormal than the two hour value, we suggest checking at one hour. If the two hour number is the prime mischief maker we then advise testing two hours after the meal.
In scientific experiments the timer starts at the beginning of the infusion so most clinics advise to time the test from the first bite.  If you start breakfast at 7.45 AM, your one hour test should be at 8.45 AM.  This works fine if the meal is short as a breakfast or lunch will often be but what about if it is a protracted dinner. The meal starts at 6.15 PM but does not finish until 7.15.  In this case we recommend taking the half way point during the meal as the start time, in this example of 6.15 to 7.15 PM we would say 6.45 PM and if testing at two hours then test at 8.45 PM.  I am not sure there is an absolute right answer but this practical approach works.