The Glucose Test in Pregnancy: One-Step, Two-Step, Results and What to Eat

What is the glucose test in pregnancy?

The glucose test is a blood test, usually done between 24 and 28 weeks, that checks how your body handles a sugary drink in order to find gestational diabetes. ACOG says all pregnant women should be screened for gestational diabetes, and the NHS notes that it usually causes no symptoms, so most cases are only found through testing.

In the US, the most common approach is a two-step test: a 1-hour screening drink, then a longer fasting test only if the first result is high. In the UK, the NHS offers a 2-hour test only to people with risk factors.

When do you take the glucose test?

MedlinePlus says most pregnant people take the glucose test between 24 and 28 weeks of pregnancy. ACOG says it might be done in the first trimester if you have risk factors for diabetes or had gestational diabetes in a past pregnancy. The US Preventive Services Task Force recommends screening at 24 weeks or later and says it can happen after 28 weeks if you start prenatal care late.

ACOG lists these risk factors: being overweight or obese, being physically inactive, gestational diabetes in a previous pregnancy, a previous baby weighing 9 pounds or more, high blood pressure, a history of heart disease and polycystic ovary syndrome (PCOS). ACOG adds that gestational diabetes can also develop in women with no risk factors.

In England, the NICE guideline says to offer a 2-hour glucose tolerance test at 24 to 28 weeks to anyone with a BMI of 30 or more, a previous baby weighing about 10 lb (4.5 kg) or more, a parent, sibling or child with diabetes, or an ethnic background with a high rate of diabetes. The NHS website also lists being over 40 and having had weight-loss surgery. The NHS says your midwife or doctor asks about these at the booking appointment, the UK name for your first prenatal appointment, at around 8 to 12 weeks.

If you had gestational diabetes before, NICE says to offer early self-monitoring of blood sugar, or the 2-hour test as soon as possible after booking and again at 24 to 28 weeks if the first is normal.

What is the difference between the one-step and two-step glucose test?

The two-step test starts with a short screening drink and only moves to a diagnostic test if that result is high, while the one-step test goes straight to the diagnostic 2-hour test. The US Preventive Services Task Force's 2021 recommendation says the two-step approach is the most common in the US, and that ACOG and the National Institutes of Health recommend it. It says the American Diabetes Association recommends either.

The task force says the two-step 3-hour test needs 2 or more high values for a diagnosis, and the one-step test needs only 1. It found that the one-step criteria pick up more cases of gestational diabetes, found no difference between the two approaches in many outcomes for you and your baby, and does not recommend one specific test.

NICE advises against using the glucose challenge test to assess risk and uses the 75 g 2-hour test for anyone with risk factors.

What do the glucose test results mean?

A high result on the 1-hour screen means you need the longer test, and only that longer test diagnoses gestational diabetes. MedlinePlus, from the US National Library of Medicine, says a result of 140 mg/dL (7.8 mmol/L) or less after 1 hour is normal, and that about 2 out of 3 people who go on to the follow-up test do not have gestational diabetes. Cleveland Clinic notes that some providers use 135 mg/dL as their cut-off.

MedlinePlus gives these abnormal values, at or above, for the 3-hour test: fasting 95 mg/dL (5.3 mmol/L), 1 hour 180 mg/dL (10.0 mmol/L), 2 hours 155 mg/dL (8.6 mmol/L) and 3 hours 140 mg/dL (7.8 mmol/L). For the one-step 2-hour test they are fasting 92 mg/dL (5.1 mmol/L), 1 hour 180 mg/dL (10.0 mmol/L) and 2 hours 153 mg/dL (8.5 mmol/L). Cleveland Clinic says that with only one high value on the 3-hour test, your provider may test you again in a few weeks.

In the UK, NICE says gestational diabetes is diagnosed with a fasting level of 5.6 mmol/L (about 101 mg/dL) or more, or a 2-hour level of 7.8 mmol/L (about 141 mg/dL) or more. Cleveland Clinic says most people get screening results within one or two days. If the test does show gestational diabetes, ACOG says a healthy diet and regular exercise control blood sugar for many women, and some need medicine as well.

What should you eat before the glucose test?

Before the 1-hour screening test you can eat normally. MedlinePlus says you do not need to prepare or change your diet, and Cleveland Clinic says most providers recommend avoiding a high-sugar breakfast such as pancakes, donuts or juice. It notes that a large sugary lunch may affect the result if your test is later in the day. Cleveland Clinic also says you cannot eat or drink from the glucose drink until the blood draw, though a few sips of water are usually fine.

Before a 2-hour or 3-hour glucose tolerance test you need to fast. MedlinePlus says to have nothing but sips of water for 8 to 14 hours beforehand and to eat your normal food in the days before. The NHS says 8 to 10 hours without food or drink for its test, though water is usually allowed. Cleveland Clinic says your provider may ask you to eat at least 150 grams of carbohydrate a day for three days before the 3-hour test. MedlinePlus suggests asking your provider whether any of your medicines can affect the result.

What is the glucose test like?

The glucose test means drinking a very sweet liquid within about five minutes and having blood taken, Cleveland Clinic says. MedlinePlus says most people have no side effects, though some feel nauseated, sweaty or lightheaded after the drink. On the 3-hour test you cannot leave between blood draws, so Cleveland Clinic suggests bringing something to read.

When should you call your doctor or midwife?

• You have not been offered a glucose test. ACOG says all pregnant women should be screened, and the NHS says anyone with a risk factor should be offered screening.
• You had gestational diabetes in a previous pregnancy. ACOG says testing may be done in the first trimester, and NICE advises testing soon after booking.
• You have signs of high blood sugar, such as increased thirst, peeing more than usual, a dry mouth, tiredness, blurred eyesight, or genital itching or thrush. The NHS notes that some of these are common in pregnancy anyway and asks you to speak to your midwife or doctor if you are worried.
• Before the test, if you take medicines that might affect the result.