From the Guidelines
The optimal time to test for gestational diabetes is between 24 and 28 weeks of pregnancy. This screening is typically done using a glucose challenge test or an oral glucose tolerance test during this timeframe. The 24-28 week window is recommended because it represents the period when insulin resistance naturally increases due to placental hormones, making it the most effective time to identify women who cannot produce enough insulin to overcome this resistance. Testing earlier may miss cases that develop later, while testing much later could delay necessary interventions. For women with high-risk factors such as previous gestational diabetes, obesity, advanced maternal age, family history of diabetes, or belonging to certain ethnic groups with higher diabetes prevalence, earlier screening may be recommended, sometimes as early as the first prenatal visit and again at the standard 24-28 week timeframe if the initial test is negative. The screening is important because undiagnosed and untreated gestational diabetes can lead to complications for both mother and baby, including excessive birth weight, preterm birth, respiratory distress syndrome, and increased risk of type 2 diabetes later in life 1, 2, 3, 4, 5, 6, 7, 8, 9.
Some key points to consider:
- The USPSTF recommends screening for gestational diabetes after 24 weeks of gestation to reduce maternal and fetal complications 2.
- The American Diabetes Association endorses glucose testing for GDM in all pregnant women who do not have a prepregnancy diagnosis of diabetes between 24 and 28 weeks of gestation using a 75-g 2-hour OGTT with thresholds proposed by the International Association of Diabetes and Pregnancy Study Groups 4.
- Women with high-risk factors may benefit from earlier screening, but the evidence on the benefits and harms of screening before 24 weeks of gestation is insufficient 1, 3.
- The 50-g OGCT is a common screening test, and a threshold of 140 mg/dl identifies 80% of women with GDM, while a threshold of 130 mg/dl further increases the sensitivity to 90% 9.
Overall, the evidence supports screening for gestational diabetes between 24 and 28 weeks of pregnancy, with earlier screening considered for women with high-risk factors.
From the Research
Optimal Time for Testing Gestational Diabetes
The optimal time for testing gestational diabetes is between 24 and 28 weeks of pregnancy.
- This period is recommended because glucose tolerance deteriorates during this time 10.
- A one-step approach using a 75g oral glucose tolerance test (OGTT) is recommended for screening and diagnosis during this period 11, 12.
- The 75g OGTT has several advantages, including faster diagnosis time, better tolerance, and easier memorization 11, 12.
- Alternative methods, such as measuring fasting blood glucose, HbA1c, fructosamine, glycosuria, or random and postprandial plasma glucose, are not recommended due to limited studies and variable criteria 11, 12.
Screening Outside the Standard Period
- Early screening may be considered for women with risk factors, such as a history of type 2 diabetes or gestational diabetes in a previous pregnancy.
- However, the benefit of early screening has not been supported by prospective studies, and oral glucose tolerance testing during early pregnancy is not currently recommended 10.
- Screening for gestational diabetes should be performed between 24 and 28 weeks of pregnancy, regardless of the recommended screening policy 10.
- There is no evidence to support subsequent screening for gestational diabetes at a later stage 10.
- A study found that women who performed the oral glucose tolerance test after 28 weeks had lower rates of gestational diabetes and type 2 diabetes, but once gestational diabetes was diagnosed, the risk of type 2 diabetes was increased regardless of the timing of the test 13.