There's a phenomenon during pregnancy that causes "insulin resistance," which means that although a pregnant woman makes insulin--it's just that it's lousier insulin. It simply doesn't react to receptors at the cellular level as well, meaning it takes more to do what insulin is supposed to do-–bring sugar into the cells for energy. Being overweight makes this worse. In pregnancy, when the insulin made just can't do the job anymore, we call this GDM (Gestational Diabetes Mellitus). Since the insulin isn't as powerful, a diet low in sugar and carbohydrates will mean less sugar in the system; less sugar in the system means less left over from the faulty insulin chemistry. This is what is meant by diet-controlled gestational diabetes and actually works fairly well with this condition.
Because diet does work well, very few women need insulin injections with GDM. Sometimes, as many as 1 out of 10 - 20 pregnancies will have GDM. It used to be a sneaky disease until we started screening all pregnancies with the O'Sullivan test (a 1-hour blood glucose determination after a sugar drink). Out of those with an abnormal test, a full 3-hour glucose tolerance test (a fasting, followed by 3 subsequent sugar determinations after a sugar drink) will then pick out the real GDM patients. GDM can have the same complication as "regular" diabetes -- large babies -- so it is important to manage it aggressively.
And then there are the "real" diabetics.
This isn't making insulin that lackluster. This is actually not making enough insulin at all. When there isn't enough insulin to bring sugar from the blood stream (your blood stream is what you eat) into the tissue, it builds up in the blood stream, gunking up the works. This leads to damage of the blood supply to one's organs, resulting in kidney damage, eye damage, etc.
Women who are diabetic when they conceive are at twice the risk for abnormal fetuses, even when their blood sugars are well controlled. But even with this doubled risk, the chances of having a baby with congenital abnormalities is only 4 to 5 out of a hundred.
But diabetic women who have blood sugars that aren't well controlled have a staggering increase in their risk-–by about ten times what their normal risk would be for a well-controlled diabetic state. So the real deal-breaker here is good control before conception. Luckily, there's a test that can tell how well the diabetes has been controlled. It is called an HbA1c (Hemoglobin A1C) and measures how much sugar is "stuck" to a certain hemoglobin molecule. The beauty of this relationship between hemoglobin and glucose is that it's a firm interaction-–meaning that it reflects how well the diabetes has been in control for a long time, usually months.
So a normal HbA1c in the first trimester will be a very reassuring test for a pregnant diabetic patient. And the risk of congenital abnormalities and miscarriage is directly related to how abnormally high the HbA1c is. This makes it, besides the serum glucose measurement, the most important prenatal (and preconception!) test in diabetic pregnancy.
But even with good control preconception and during the first trimester, the two natural enemies, pregnancy and diabetes, begin to fight it out. The very thing that can cause that normal variation known as GDM can make insulin-dependent diabetes harder to manage, too, by driving up the insulin needs. This means that no "set" dosage of insulin can be established with expectations of the pregnancy sugar control to be on automatic pilot. It doesn't work that way, and pregnancy + diabetes is usually a continuing medical challenge always at red alert.
Large babies make for more difficult vaginal deliveries. Besides the risk of cephalopelvic dysproportion (baby's to big to fit out), and shoulder dystocia (head delivers but shoulders get stuck), there is also increased risk of placental abruption (premature separation). Since the C-section rate is higher in diabetic patients for all of the above reasons, it's important to know that Cesarean delivery is not the perfect answer to a pregnant diabetic's problems. Diabetic women don't heal well after surgery and their chances of infection are much greater.
In spite of all of these concerns, a woman whose sugars are well controlled can stack the deck in her favor, especially if she is evaluated preconception. But diabetes is a definite problem in pregnancy that requires diligence on the part of the obstetrician and strict compliance on the part of the patient.