Gestational Diabetes Screening: Choosing Between One-Step And Two-Step Testing

Gestational diabetes mellitus (GDM) is glucose intolerance first recognized during pregnancy. It can develop without obvious symptoms, yet it is associated with fetal overgrowth, hypertensive disorders, cesarean birth, shoulder dystocia, neonatal hypoglycemia, and a higher long-term risk of type 2 diabetes for the mother. Screening provides an opportunity to identify abnormal glucose regulation early enough for nutrition counseling, glucose monitoring, medication when needed, and closer obstetric care.

The main diagnostic pathways are the one-step 75-gram oral glucose tolerance test (OGTT) and the two-step strategy, which begins with a nonfasting 50-gram glucose challenge and continues with a diagnostic 100-gram OGTT when the screening result is elevated. The choice is influenced by professional guidance, local resources, patient preferences, and the balance between detecting more cases and limiting unnecessary treatment.

Interest in perinatal research has kept these questions central to specialist meetings and clinical education. The FAOPS 2020 archive reflects the broader scientific setting in which maternal glucose, fetal development, neonatal outcomes, and pregnancy management are studied together.

Why Gestational Diabetes Screening Matters

Pregnancy naturally increases insulin resistance, particularly during the second and third trimesters. The placenta produces hormones that help direct glucose toward the fetus, but the maternal pancreas may not increase insulin production enough to compensate. When this adaptation is inadequate, blood glucose rises. Risk is greater with previous GDM, a history of a large baby, obesity, polycystic ovary syndrome, a first-degree relative with diabetes, advanced maternal age, or belonging to a population with a high prevalence of type 2 diabetes.

Untreated hyperglycemia can contribute to excessive fetal growth and increased amniotic fluid. The newborn may experience low blood glucose after birth because fetal insulin production remains high after placental glucose delivery stops. Maternal glucose control also affects decisions about timing of delivery, fetal surveillance, and neonatal observation. Screening is therefore part of a larger care pathway rather than an isolated laboratory event.

Most organizations recommend testing at 24 to 28 weeks for patients who do not have known diabetes. Early pregnancy assessment is appropriate for people with substantial risk factors, previous GDM, or symptoms and laboratory findings suggestive of overt diabetes. A markedly elevated early result may represent previously undiagnosed diabetes rather than typical gestational diabetes and requires a different management approach.

How The One-Step Method Works

The one-step approach uses a fasting 75-gram OGTT. The patient has a fasting blood sample, drinks the glucose solution, and provides additional samples one and two hours later. Under commonly used International Association of Diabetes and Pregnancy Study Groups (IADPSG) or World Health Organization thresholds, GDM is diagnosed when any one value reaches or exceeds the following levels: fasting 92 mg/dL (5.1 mmol/L), one hour 180 mg/dL (10.0 mmol/L), or two hours 153 mg/dL (8.5 mmol/L).

A single abnormal value is sufficient. This makes the process relatively direct: there is no preliminary challenge test and no second appointment for patients who screen positive. It also increases diagnostic sensitivity, identifying milder degrees of hyperglycemia that may be missed by a more selective pathway. Patients typically need to fast overnight, remain at the testing site for about two hours, and avoid unusual physical activity or eating during the procedure.

The one-step method can be attractive where reducing missed diagnoses is a priority. Its limitations include a greater number of positive results, additional counseling and monitoring, and possible treatment of cases whose clinical significance is modest. Fasting requirements can also create practical difficulties, particularly for people with nausea, demanding work schedules, limited transportation, or other children to care for.

How The Two-Step Method Works

The two-step strategy starts with a 50-gram, one-hour glucose challenge test. Fasting is generally not required. If the one-hour plasma glucose exceeds the threshold selected by the practice—often 130, 135, or 140 mg/dL—the patient returns for a diagnostic 100-gram, three-hour OGTT after an overnight fast. The lower cutoff improves sensitivity, while the higher cutoff reduces the number of follow-up tests.

The diagnostic 100-gram test requires fasting blood glucose followed by measurements at one, two, and three hours. With the widely used Carpenter-Coustan criteria, the thresholds are fasting 95 mg/dL (5.3 mmol/L), one hour 180 mg/dL (10.0 mmol/L), two hours 155 mg/dL (8.6 mmol/L), and three hours 140 mg/dL (7.8 mmol/L). At least two abnormal values are usually required for a diagnosis. Some practices use a different set of thresholds or accept one abnormal result, so the laboratory protocol should be documented clearly.

Compared with the one-step pathway, this approach creates a filter before the longer diagnostic test. Many patients avoid fasting and extended testing because their initial challenge result is normal. The trade-off is that some clinically relevant hyperglycemia may not be detected, especially when the preliminary cutoff is high or when two abnormal diagnostic values are required.

Comparing Clinical Trade-Offs

The two methods are based on different priorities. The one-step test aims to identify more patients with glucose levels associated with adverse pregnancy outcomes. The two-step method places greater emphasis on specificity, workload, and reserving diagnosis for more persistent or pronounced abnormalities. Neither approach eliminates the need for clinical judgment, because risk varies across populations and treatment resources.

Evidence comparing pregnancy outcomes remains complex. A method that labels more people as having GDM may improve outcomes through earlier intervention, or it may increase medicalization without a proportionate benefit for every patient diagnosed. Results also depend on how consistently patients receive nutritional support, glucose meters, medication, fetal growth assessment, and postpartum follow-up.

Selecting A Screening Pathway

Health systems should begin with the guideline adopted by their professional organization and the population they serve. A hospital with limited laboratory capacity may favor the two-step strategy because the initial challenge test is simpler and only a subset of patients needs the three-hour test. A service seeking maximum case detection may prefer the one-step OGTT, provided it can support more diagnoses and the resulting monitoring workload.

Patient-centered care is important. Explain fasting requirements, test duration, the possibility of nausea, and what an abnormal result means before the appointment. A positive screen does not automatically indicate poor health or guarantee medication. It signals that glucose regulation deserves diagnostic clarification or, when the one-step criteria are used, structured management.

The result should be interpreted alongside gestational age, symptoms, previous pregnancy history, fetal growth, and other metabolic findings. People unable to complete an OGTT because of vomiting or previous bariatric surgery need an individualized alternative arranged by the maternity and diabetes teams. Capillary glucose profiles may sometimes be used, but they should not be treated as automatically equivalent to validated laboratory thresholds.

Integrating Results With Perinatal Care

A GDM diagnosis commonly leads to nutrition counseling, physical activity guidance appropriate for pregnancy, and self-monitoring of fasting and post-meal glucose. Targets vary by guideline, but many programs use fasting values below 95 mg/dL and one-hour post-meal values below 140 mg/dL. If lifestyle measures do not achieve targets, insulin is a well-established treatment; some settings also use metformin after discussing benefits, limitations, and local guidance.

Fetal growth assessment and antenatal surveillance are tailored to the severity of hyperglycemia, medication use, comorbidities, and obstetric history. Perinatal assessment should remain focused on the whole pregnancy rather than glucose alone. For example, specialist resources on fetal MRI guidance illustrate how imaging may complement clinical evaluation when fetal structural concerns arise, although MRI is not a routine test for GDM itself.

After birth, insulin resistance often improves, but the underlying metabolic risk does not disappear. A 75-gram OGTT at four to twelve weeks postpartum is commonly recommended because fasting glucose alone can miss impaired glucose tolerance. Ongoing diabetes screening every one to three years, weight management, healthy eating, physical activity, and preconception counseling in a future pregnancy help reduce long-term risk.

Practical Recommendations For Safer Screening

• Use one clearly documented protocol, including glucose thresholds, fasting instructions, and the number of abnormal values required.
• Offer early diabetes assessment to patients with substantial risk factors, then repeat standard GDM screening at 24 to 28 weeks when early testing is normal.
• Explain the difference between a preliminary screen and a diagnostic OGTT so patients understand why a second test may be necessary.
• Link abnormal results to prompt nutrition education, glucose monitoring, medication assessment, and appropriate fetal and neonatal planning.
• Record the diagnosis in the discharge summary and arrange postpartum glucose testing before the patient leaves maternity care.

Reliable implementation depends on more than selecting one set of numbers. Laboratories need validated assays, staff need consistent instructions, and patients need access to interpreters, transportation support, nutrition services, and follow-up appointments. Auditing test completion, positive rates, treatment initiation, pregnancy outcomes, and postpartum attendance can reveal whether the selected pathway is working fairly and effectively.

Clinicians and perinatal teams can use the evidence behind both approaches to create a screening pathway that fits their patients, resources, and outcomes. Clear counseling and dependable follow-up turn an OGTT result into meaningful protection for maternal health and newborn well-being.