Some women with diabetes who become pregnant or women who have been diagnosed with gestational diabetes become worried when they read of women with diabetes delivering huge babies. In a large Scandinavian study of over 10,000 women with GDM only 8% had babies over 4.5 kg (nearly 10 lbs) and we know treatment will lessen this chance. In another large study of babies of women with Type 1 diabetes the average birth weight was 205 g (7 oz) heavier than the general population. There is a problem but not as big as some expect; so why does diabetes cause big babies? Sometimes in a simplistic manner it is stated that the high sugar goes into the baby and is converted into fat. It is more complex (see below) but what is important to remember is that diabetes is only one of many contributors to the size of the baby, an important one, and one that we can do something about but it is not the only thing to determine the size of the baby.
An increase in the baby’s birth weight is related to:
The mom’s weight – heavier women have bigger babies.
The mom’s weight gain in the pregnancy – the more gained the heavier the baby, blog 12th July 2011.
The size of the dad – the taller and bigger the more the baby will weigh, it’s not all due to mom.
The gender of the baby – boys weigh more than girls.
The duration of the pregnancy- a 39 week baby is heavier than a 37 week baby.
Ethinicity – some races have larger children.
And – The blood sugar.
For women with gestational diabetes the one factor of all of these that we can focus on is the blood sugar but in many cases the blood sugar is not the most important thing in determining the birth weight. Hence we may see some women who have great blood sugar control but yet have big babies and other women with awful sugars whose baby is fine. Some babies just may be very sensitive to high glucose levels but I suspect that when really large babies occur that there are factors over and above glucose alone involved. Overall there is a relationship, the higher the blood sugar the bigger the baby and the better the blood sugar the smaller the baby, and so in an individual case we just try our best to keep the sugar controlled. Some diabetes doctors think any slight rise in blood sugar is bad, I think there is a bigger picture.
When the mom has high blood sugar then some of the sugar passes across the placenta and into the baby. The baby’s own pancreas makes insulin to handle the higher blood sugar, the insulin in the mom does not cross the placenta. Although insulin is a a growth factor and could conceivably cause growth of the baby, it may be that since insulin lowers the binding proteins for growth factors in the baby’s circulation, this allows more of the growth factors to be “free” and may be the cause the baby’s extra growth. I actually think this is only part of the story and there may be other hormones, yet to be discovered that cause the big baby.
In the mean time, it is important to control the blood sugar as best one can but I doubt if the odd slight rise of blood sugar does a lot of harm by itself, it is more when the sugars are consistently high that there is concern.