Screening for Gestational Diabetes

In this post, I compare guidelines for testing for gestational diabetes. 4 – 9 % of pregnant people have gestational diabetes. Women with GDM are at a higher risk for gestational hypertension and cesarean. Their babies are at higher risk of macrosomia (being > 9.9 pounds), hypoglycemia, jaundice, cesarean delivery, and shoulder dystocia.

Types of Tests Used

GCT = A glucose challenge test is a screening test, where an expectant parent drinks 50 g of a sugary beverage (or eats a prescribed sugary snack such as white toast with honey), and then one hour later blood is drawn to test her blood glucose levels. If they are high (above the threshold listed below), she will then have a GTT.

GTT = A glucose tolerance test is a diagnostic test. After fasting for 8 hours, a woman drinks either 75 g or 100 g of a sugary beverage, and then has her blood drawn at 1 hour, 2 hours, and 3 hours. If her blood glucose levels are high, then gestational diabetes is diagnosed.

Who is at risk (aka categories of risk):

High risk: Risk factors that increase a woman’s risk for developing GDM include obesity (BMI >30), increased maternal age (>35), history of GDM, family history of diabetes, and belonging to an ethnic group with increased risk for type 2 diabetes. [NICE defines high risk as BMI > 30, previous baby over 9.9 pounds, previous GDM, first-degree relative with diabetes, South Asian, black Caribbean, Middle Eastern. PCOS.]

Average risk: not high risk, not low risk.

Low risk: under 25 years, normal BMI before pregnancy, member of ethnic group with low prevalence of GDM, no known diabetes in first degree relatives

When to test:

ADA says screen high risk women as early as possible, and re-test at 24- 28 weeks if first screen was negative. Screen average risk women at 24- 28 weeks. For low risk asymptomatic women, no screening required.

ACOG: Screen all women at 24 – 28 weeks

CDA: Screen all women at 24 – 28 weeks (Optional: Screen higher risk women earlier and again at 24 – 28)

NICE: Do a GTT at 16–18 weeks if prior GDM; at 24–28 weeks if risk factors

USPSTF says there is not sufficient evidence for screening before 24 weeks. Should screen after 24 weeks.

How to test:

ADA – 1 step: 75 g fasting GTT may be cost-effective for high risk patients or populations. Or 2 step: 50 g GCT. Then, if indicated, 100 g GTT

ACOG: 2 step –-50 g GCT, then fasting 100 GTT

CDA: Preferred 2 step – 50 g GCT, then if indicated 75 g GTT. Alternate 1 step: 75 g GTT

NICE: 75 g GTT (but only test if there are risk factors)

On GCT results, what threshold indicates need for GTT diagnostic testing?

ADA / ACOG / CDA: If <140 mg/dl, no further testing is indicated. If > 140, then GTT should be done. ACOG says 135 should be threshold in women at high risk for GDM.

If the threshold was set at 130, then you would catch 90% of cases of GDM, vs. the 80% of cases you catch when setting threshold at 140. However, at 130, there would be many more false positives – women diagnosed and treated for GDM who did not have it. [Note: None of these guidelines share what the false positive rate is.]

On GTT results, what threshold indicates a diagnosis of gestational diabetes?

Fasting 1 hour 2 hour
ADA/CDA 1 step 75 g GTT ≥ 92 mg/dl (5.1 mmol/L) ≥ 180 (10) ≥ 153 (8.5)
ADA / ACOG 2 step 100 g GTT ≥ 95 mg/dl ≥ 180 ≥ 140
CDA 75 g GTT ≥ 95 (5.3) ≥ 190 (10.6) ≥ 153 (9.0)
NICE 75 g GTT ≥ 126 (7.0) ≥ 140 (7.8)

Can an expectant parent do anything to decrease her risk of a false positive result?

About 15 – 20% of expectant parents test positive on GCT. Only 4 – 9% are diagnosed with GDM after a GTT. (Doing only the one-step GTT increases the rate of diagnosis, which likely means there are more false positives with the one step than with the two step method.)

To reduce false positives on a GCT. Eat healthy, non-sugary meals in the days before the test. Be well-rested and relaxed on the day of the test. Eat a small, healthy non-sugary meal one hour before test, then walk around to let your body metabolize it. In the hour between drinking the glucose and having blood taken, do not drink tea, coffee, soda, etc.; eat sugary food, or smoke cigarettes.


ACOG 2013

ADA, American Diabetes Association, 2003

CDA: Canadian Diabetes Association, 2013




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