Just how low can a normal blood sugar go in pregnancy?

Recently in the clinic I was asked “Is 3.8 mmol/l (68 mgs/dl) two hours after my lunch too low? The blood sugar normally varies though out the day:  lower before breakfast, peaking just over an hour after a meal and coming back down by two hours.  Firstly, sugars at this level do not harm the baby but let us look at what normal sugars in people without GDM are and then we can look at the situation in gestational diabetes and for those with pre-existing diabetes, either Type 1 or Type 2 diabetes.
During the night the baby is consuming glucose so the before breakfast or fasting sugar is lower.  Based on studies in pregnant women who did not have diabetes (see GDM-Research 1st Sept. 2011) the usual sugar values found are shown in the Table with the range being the levels that more than 95% of women had sugar readings between:

These are the numbers are in pregnant women without diabetes. When dealing with women with GDM the important issue is what treatment are they on. If on diet alone then numbers falling below this typical range are likely not a concern.  Their meter may be inaccurate and reading low (see GDM-Research 25th Mar 2011), this just may be “normal” for them, perhaps the release of insulin from their pancreas is a little sluggish and peaks later causing  the lower numbers.  If the low readings continue or the mom feels unwell with them then an earlier snack may help. However if she is taking treatment although the same explanations may apply, the most likely thing is the therapy is too strong so a reduction in insulin or oral hypoglycaemic medicines (metformin or glyburide) is in order.
For pregnant women with Type 1 diabetes and nearly all with Type 2 diabetes, they are usually on insulin so lowish readings carry the risk of dropping further and the person becoming hypoglycemic.  Thus a fasting sugar under 3.5 mmol/l (63 mgs/dl) should usually prompt a reduction in the long acting insulin working overnight.  Values less than 5.0 mmol/l (90 mgs/dl) at one hour after the meal or less than 4.5 mmol/l (81 mgs/l) two hours after a meal would in our clinic lead to a decrease of the premeal insulin if there was no other ready explanation.
So low sugar readings have to be taken in context. A sugar of is 3.8 mmol/l (68 mgs/dl) two hours after lunch in someone on no treatment is likely fine and all will be okay if left alone. A similar sugar in someone on diabetes treatment may then go down more and so the woman should take a snack.  If there is no other reason for the low number (eg smaller meal or vomited, walked a lot), she should likely reduce her insulin or diabetes medicine.