Who is tested, and when

Gestational diabetes test results come from a screening offered to every pregnant woman. The US Preventive Services Task Force recommends it at 24 weeks or later, and ACOG places it between 24 and 28 weeks. If you have risk factors, your blood sugar is checked earlier, often at the first visit, then again later if that first check is normal. It is one of the prenatal blood tests on the usual schedule.

Gestational diabetes appears when pregnancy hormones make insulin work less well and the body cannot make enough extra insulin to keep up. The Task Force estimates that it affects 5.8% to 9.2% of US pregnancies with the usual criteria. It can happen to anyone, which is why the test is not reserved for people at risk.

  1. WeightOverweight or obesity, and physical inactivity.
  2. Past pregnanciesGestational diabetes before, or a baby of 9 pounds or more.
  3. Health historyHigh blood pressure, heart disease or polycystic ovary syndrome.
  4. BackgroundHigher rates in women of African, Asian, Hispanic, Native American and Pacific Island descent.

The two-step method: 1 hour, then 3 hours

Most US practices start with the glucose challenge test. You do not need to fast. You drink a sweet liquid containing 50 grams of glucose, and blood is drawn one hour later. Some people feel nauseated, sweaty or lightheaded after the drink, and serious side effects are very uncommon. Eat normally in the days before, and ask whether any of your medicines can affect the result.

If that number is too high, you come back for a three-hour test. This one is done fasting: nothing to eat or drink but sips of water for 8 to 14 hours. Blood is drawn before a 100-gram glucose drink and then every hour for three hours. If the challenge result is 200 mg/dL or more, the NIDDK notes that you may have type 2 diabetes rather than a pregnancy-related form.

TestTime of drawResult that counts as high
1-hour challenge (50 g)1 hourAbove 130, 135 or 140 mg/dL, depending on the practice
3-hour test (100 g)Fasting95 mg/dL (5.3 mmol/L) or more
3-hour test (100 g)1 hour180 mg/dL (10.0 mmol/L) or more
3-hour test (100 g)2 hours155 mg/dL (8.6 mmol/L) or more
3-hour test (100 g)3 hours140 mg/dL (7.8 mmol/L) or more
One-step test (75 g)Fasting / 1 h / 2 h92 / 180 / 153 mg/dL or more

How to read the 3-hour result

On the three-hour test, gestational diabetes is diagnosed when two or more of the four values reach the cutoffs. ACOG accepts these values, known as the Carpenter–Coustan criteria, and a slightly higher set from the National Diabetes Data Group; your lab report or your practice will say which one it uses.

If only one value is high, you do not have the diagnosis, but your provider may suggest changing some of the foods you eat and testing you again later. That single value is still worth a conversation, because the large HAPO study of 25,505 pregnant women found that the risks for the baby rise steadily with the mother’s glucose, with no clear line where they suddenly start.

The one-step method

Some practices skip the challenge and go straight to a two-hour fasting test with a 75-gram drink, the method proposed by an international consensus in 2010. Here a single value at or above 92 mg/dL fasting, 180 at one hour or 153 at two hours is enough for the diagnosis.

The two methods do not label the same number of people. In a trial of 23,792 women, the one-step method diagnosed gestational diabetes in 16.5% of them against 8.5% with the two-step method, with no difference in large babies, birth complications, preeclampsia or cesarean births. ACOG accepts both approaches, and the US Preventive Services Task Force leaves the choice open.

After a positive result

A diagnosis means closer follow-up, not a failure. Many women manage their blood sugar with food changes and regular activity, checking it with a meter at home; some need medicine. Controlling it lowers the risks for you and the baby, which include a very large baby, a harder birth and low blood sugar in the newborn.

Gestational diabetes usually goes away after the birth, but it raises the chance of diabetes later in life. ACOG advises a blood test 4 to 12 weeks after delivery, then a diabetes check every 1 to 3 years if that test is normal. You can follow those later results with the guide to blood sugar and other routine markers.