Why Babies from Diabetic Moms are Bigger

Patient Expert

Lately I've been thinking a lot about why babies of diabetics tend to grow bigger than those of non-diabetic women.   This started last week when I had my 35 week growth ultrasound.   The primary purpose of that examination is to gauge how large the baby is and make any delivery plans that would be necessary should the baby be quite large at that stage.

Luckily, this little one is only in the 60% range at 5 pounds 15 ounces, which puts him or her on track to weigh about 8 to 8 ½ pounds at birth.   The doctor who read the growth measurements and ultrasound photos came in to say, "Well, it's not going to be a 10 pound baby"   Which got me thinking, are 10 pound babies common for diabetic women?

The next day I saw a friend of mine who is pregnant with her first baby and due the same day as me.   She has gained an average amount of weight and looks about 35 weeks pregnant.   We compared growth measurements for our babies and I learned that her baby is in the 30% and only weighs about 4 pounds!

Here I am, eating a fairly low carbohydrate diet, swimming several times a week, and having gained less than 15 pounds and my baby outweighs hers by 2 pounds!

Now I understand that every woman and baby is different, so this one comparison doesn't mean all that much.   However, it made me wonder what exactly causes the baby of a diabetic mother to grow bigger than normal?

A couple days later I went in for one of my bi-weekly non-stress tests.   I've gotten to know the nurse who performs these tests pretty well as she also took care of Sienna and me during the last month of her pregnancy.   I asked her, "So, what causes babies of diabetics to grow faster than normal?   Is it the total calories or carbohydrates consumed or just the impact of high blood glucose levels?"   She replied, unequivocally that it's the out of range blood sugar levels that cause the baby to grow.

My understanding is that when my blood sugar is high, there's superfluous glucose directed toward the baby which causes growth.   This made me feel better about the bagel I'd had the day before, since my blood sugar remained under 140 mg/dl after that indulgence.  The relationship between carbohydrate intake and growth is, of course, that eating higher carb meals or snacks tends to spike blood sugar levels which cause the baby to grow.

So, as the holiday season is in full swing, I'm going to cut myself some slack about consuming the occasional Christmas cookie or goodie.   I'll just plan ahead and test my blood sugar frequently to avoid those high blood sugars; because I'm glad the baby won't be 10 pounds, but I'd like to avoid a 9 pound baby too!