There are a few good reasons why each prenatal care visit involves the collection of a urine sample… and it’s not just to check your contortionist abilities while pregnant! In addition to screening for infections or protein, your healthcare providers are also testing your urine for the presence of sugar (i.e. glucose), known as “glycosuria”.
What causes glycosuria?
Glucose is not typically found in urine. The hormone insulin causes glucose to move from your blood into cells in your body, such as fat and muscle, to be used for energy. Any remaining glucose in your blood is normally filtered and reabsorbed back into your body by your kidneys, the bean-shaped organs in the back of your abdomen that are in charge of producing waste (i.e. urine). If your blood glucose levels are too high and your kidneys can’t fully reabsorb all of the sugar, then glucose will be present in your urine. Glycosuria is a warning sign of diabetes, and when diabetes develops during pregnancy it is called gestational diabetes. Therefore, when your doctors are testing your urine for glucose, they are screening you for gestational diabetes.
False positives may occur!
It’s important to note that the urine glucose tests at your regular prenatal care appointments are only designed to serve as a screen for gestational diabetes, and more thorough tests are required for a full diagnosis. In fact, it is estimated that over half of all pregnant women have glucose in their urine at some point during pregnancy, but only 6-7% of all pregnant women receive a diagnosis of gestational diabetes…. So don’t panic if your doctor says there is glucose in your urine! An overly sweet breakfast that day could be to blame.
Comprehensive glucose tests are required to diagnose diabetes
Most doctors will order the glucose challenge screening between 24 and 28 weeks of pregnancy. If you have glycosuria or other signs of gestational diabetes (like excessive thirst, fatigue, or sensory disturbances), or if you have risk factors such as a history of diabetes, a high BMI, or are over 30 years old, then your doctor might ask you to complete the test earlier in your pregnancy. At this test you will consume a drink with a very high sugar content – 50 grams, or the equivalent of 28 jelly beans! – which causes a sharp spike in blood glucose levels and tests your body’s ability to correctly process the sugar. After 1 hour your blood will be drawn, and the glucose content in your blood will be measured. If your levels are under a certain threshold (typically 135-140 mg/dL) then you won’t be diagnosed with gestational diabetes. If, however, your number is higher than the threshold range, then you will need to take a second test for a confirmed gestational diabetes diagnosis.
The second test, the glucose tolerance test, is longer and more involved than the glucose screening challenge. At this test, your blood will be drawn at baseline to measure your fasted blood glucose levels (which means no eating or drinking anything but water for 12 hours prior). You will then consume a sugary drink and have blood drawn at 1-, 2-, and 3-hour time points. If your blood glucose levels are too high at 2 or more of the 4 time points, then you will be diagnosed with gestational diabetes. A diagnosis of gestational diabetes generally requires careful monitoring of blood glucose levels, along with a healthy diet low in sugar and regular exercise. The good news is that most cases of gestational diabetes resolve after delivery!
In very few cases, a rare condition called renal glycosuria may be to blame for a positive urine glucose test. This hereditary condition involves glycosuria when blood glucose levels are normal, and is independent from insulin-mediated changes in blood glucose levels that are typical in gestational diabetes. Your doctor can easily rule this out based on your blood and urine test results.
Some medications might also cause glucose to appear in your urine in the absence of diabetes, such as certain antibiotics (e.g. amoxicillin, ampicillin, nitrofurantoin) and muscle relaxants (e.g. metaxalone). Tell your doctor about any medications you are currently taking, and do not stop taking any medications without speaking to your doctor first.
The increased risk of developing diabetes during pregnancy explains why screens for glycosuria are so important. Consuming low-sugar and high-fiber foods is critical for women with gestational diabetes, but all expecting mothers and their babies will benefit from a healthy diet.