Approach to diabetes in pregnancy: Clinical sciences

Diabetes is one of the most common medical complications in pregnancy. Patients with diabetes in pregnancy are more likely to develop preeclampsia and undergo cesarean delivery. Additionally, higher glucose levels cross the placenta, resulting in an increased glucose supply to the fetus. The fetal pancreas responds by producing more insulin to handle the excess glucose. Fetal hyperinsulinemia promotes increased fat accumulation, particularly in the shoulders and chest, that can cause macrosomia; shoulder dystocia; and birth trauma.

Moreover, once the umbilical cord is clamped after delivery, the maternal glucose supply is interrupted, which can lead to neonatal hypoglycemia. Most cases of diabetes in pregnancy are gestational diabetes mellitus, or GDM, which is hyperglycemia that develops during pregnancy. However, many patients don’t receive diabetes screening before pregnancy, so it can be challenging to differentiate between GDM and previously existing, or pregestational, type 1 or type 2 diabetes.

The first step in evaluating a patient who presents for diabetes screening in pregnancy is to obtain a focused history and physical exam, ideally at the initiation of prenatal care. First, you want to assess whether a patient has a previous diagnosis of either type 1 or type 2 diabetes. If they do, that’s pregestational diabetes mellitus. This is an important distinction to make, because patients with pregestational diabetes are more likely to have significant maternal and fetal complications during pregnancy, and usually require additional monitoring.

On the flip side, if your patient has no previous diagnosis of type 1 or type 2 diabetes, your next step is to assess whether they’re at high risk for GDM. A patient who is at high risk will have an elevated BMI of at least 25, or at least 23 in patients of Asian descent; plus one or more additional risk factors. These additional risk factors in history include GDM in a previous pregnancy; a first-degree relative with diabetes; previous delivery of an infant weighing at least 4,000 grams or about 9 pounds; or a personal history of polycystic ovarian syndrome or cardiovascular disease.

Some additional risk factors can be discovered on physical examination, including hypertension, or prepregnancy morbid obesity with a BMI greater than 40. Finally, additional risk factors related to labs include a hemoglobin A1c of 5.7 % or greater, and certain abnormal lipid values, like an HDL lower than 35 mg/dL and triglycerides higher than 250 mg/dL.

All pregnant patients should be screened for GDM - it’s just a matter of when, which is based on assessment of risk factors. First, let’s talk about patients who are at “average risk” for GDM; meaning they have no additional risk factors for GDM. Average risk patients are screened at 24 to 28 weeks of gestation with a 50-gram, one-hour oral glucose tolerance test. A normal test result indicates that the patient does not have gestational diabetes and can proceed with routine prenatal care.

Here’s a clinical pearl! The cut-off value for a normal one-hour glucose test varies between 130 and 140 mg/dL because there isn’t enough evidence to determine the ideal threshold to screen for gestational diabetes. Each clinical site or institution should decide which cut-off to use for screening and remain consistent throughout their practice.

Alright, whichever screening cut-off is used, if the one-hour glucose test for an average-risk patient is elevated, the patient should then undergo a 100-gram, three-hour oral glucose tolerance test. This test includes a fasting glucose measurement as well as additional measurements at 1, 2, and 3 hours after consuming the glucose load.

Here’s a high-yield fact! A commonly used set of diagnostic thresholds for the three-hour glucose test is the Carpenter and Coustan criteria, which include normal glucose values of fasting below 95, a one-hour result below 180, a two-hour result below 155, and a three-hour result below 140. Another acceptable approach would be to use glucose values established by the National Diabetes Data Group, which are fasting less than 105, a one-hour less than 190, a two-hour less than 165, and a three-hour less than 145.

Okay, back to our patient. The three-hour test is considered normal if no more than one of the four values is elevated. A normal 3-hour test result at 24 to 28 weeks means that your patient does not have gestational diabetes and can resume routine prenatal care.