Gestational diabetes mellitus (GDM) is the diagnosis of diabetes, or carbohydrate intolerance, in pregnancy. This condition complicates about 5 percent of all pregnancies, with some ethnic groups being more affected than others. There are a few important facts you should know about GDM, but you may be wondering how it is even diagnosed in the first place.

Doctors used to only screen women who were considered high risk for GDM. These were women who had a history of GDM or a strong family history of diabetes, are obese, gave birth to a large infant in the past, or are over 25, to name just a few risk factors. Taken together, only 10 percent of women would be excluded from this risk-based screening. Because of that, the American Congress of Obstetricians and Gynecologists now recommends that all pregnant women be screened to ensure no one who should be screened is missed.

The most common way to screen for GDM is by performing a one-hour oral glucose tolerance test. This involves drinking a 50-gram drink and then having your blood sugar checked one hour later. If your blood sugar is in the abnormal range, you will then need to do a confirmatory test to prove that you do in fact have GDM (as the 1-hour test is only a screening test). With this test, you need to fast but will be provided with drink a 100 g drink. You will also have your blood checked when you are fasting as well as one, two, and three hours after you your drink is consumed.

You will officially have the diagnosis of GDM if your 1-hour screening test was above 200 (there is no need to do a 3-hour test if it is this high) or if two or more values on your 3-hour test are abnormal.

Different providers will use different thresholds for what is considered an abnormal blood glucose level for the 1-hour test—some use 130 and others 140. There is no right or wrong number to use, but using the 130 value means that more women will need to do a follow-up test but will not actually have GDM.

The 1-hour test is usually done between 24–28 weeks of pregnancy. However, you may be asked to do the test as early as your first prenatal visit if you have certain risk factors (such as having GDM before, carrying twins, or if you are obese). If that early test is negative, you’ll need to repeat it again at the usual time.

Not all women can undergo traditional screening. For example, if you’ve had bariatric surgery and can’t tolerate that much sugar at once or if you repeatedly are unable to keep the sugar drink down, your doctor or midwife may recommend alternative screening such as monitoring your blood sugars over the span of a few days or a week.

Reviewed by Dr. Jen Lincoln, December 2018


  • All pregnant women should be screened for gestational diabetes (GDM).
  • The 1-hour screening test is only diagnostic for GDM if your blood sugar is over 200; otherwise, you’ll need a confirmatory 3-hour test.
  • You need to be fasting for the 3-hour test, but should not be for the 1-hour test.
  • Screening is usually done between 24-28 weeks, but may be done earlier if you are high risk for GDM.


  1. American Congress of Obstetricians and Gynecologists. Practice Bulletin #137: Gestational diabetes mellitus.


  1. I am 36weeks and have GDM. My blood sugar has been fine since I started monitoring it. I keep a log of the food I eat and nothing is different in the last week, but the glucose levels are all above the range my doctor would like to see. Is this due to the final weeks of pregnancy?

  2. I’m a Type 1 Diabetic, so fortunately, I didn’t have to take the test! But I was on a preventative study as a kid where I had to do glucose tolerance tests every 3-4 months. It was awful stuff… I don’t know if they every have the cola versions for pregnant women, but that one was my “favorite.” The lemon-lime came in second for me. They were all nasty though. And even as a diabetic, I still have a sweet tooth…just not for that mess!

  3. Why does that drink have to taste so bad?? It gave me awful heartburn. ;(

    1. I know – I’d say some people hate it, and others love it. There are different flavors so it might just be you got a yucky one? Also I think it is better when they are cold. I really like mine – it was like flat orange tang. I’ll admit, I have a real sweet tooth though!


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