Gestational diabetes test: timing, preparation and results

When is the test offered?

Routine testing is often between weeks 24 and 28. Earlier blood glucose testing may be recommended if you had gestational diabetes before or have other risk factors. A normal early result does not always replace the standard later test.

One-step and two-step testing

A one-step oral glucose tolerance test uses a fasting blood sample, a measured glucose drink and further samples over two hours. A two-step pathway begins with a one-hour glucose challenge that may not require fasting; a raised screen is followed by a longer fasting diagnostic test.

How to prepare

Use only your clinic’s instructions. For a fasting OGTT, you may need no food and only plain water for 8–12 hours, with no smoking, vaping, gum or strenuous activity. Continue prescribed medicine unless the team tells you otherwise, and bring a snack for afterwards.

During and after the test

A fasting sample is taken, you drink the glucose solution within the stated time, then wait quietly for later blood samples. Tell staff if you vomit or feel unwell because the test may need rescheduling. You can usually eat and resume normal activity afterwards.

Understanding results

Diagnostic glucose thresholds vary by country and testing pathway, so use the laboratory and maternity team’s interpretation. A positive result leads to education, glucose monitoring and a plan involving food, activity and sometimes medication. Postpartum testing is also important.