Why Is a Glucose Challenge Test Performed in Pregnancy?
What Is a Glucose Challenge Test During Pregnancy (Gestational Diabetes Screening)?
Gestational diabetes is high blood glucose first detected during pregnancy. It usually causes no symptoms. When it develops later in pregnancy, the early stage of the baby's organ development has usually already passed. Therefore, the risk of congenital anomalies is more closely associated with diabetes that existed before pregnancy or went undetected in early pregnancy.
When and how is glucose challenge testing performed?
Pregnant women without a previous diagnosis of diabetes are usually screened between 24–28 weeks of pregnancy. If there are risk factors for diabetes, the doctor may request an assessment earlier in pregnancy. If early testing is normal, repeat screening at 24–28 weeks is usually needed.
There are two commonly used screening approaches:
1-Two-step approach
50 g glucose challenge test: Fasting is not required. Blood glucose is measured one hour after drinking a glucose-containing beverage. The threshold used to interpret the result may be 130, 135 or 140 mg/dL, depending on the healthcare facility's protocol. This is a screening test; a high result alone does not establish a diagnosis of gestational diabetes.
100 g, 3-hour OGTT: This is performed if the screening result reaches the threshold. Fasting is required before the test. Diagnosis is based on the measurements taken during the test, evaluated according to the criteria used; the commonly used Carpenter–Coustan criteria generally require at least two of the four values to be at or above the threshold.
2-One-step approach
Some centres use a 75 g, 2-hour OGTT directly. This test requires fasting, and blood glucose is measured while fasting and at 1 and 2 hours. The diagnostic thresholds and test protocol differ from those of the two-step approach. The method used depends on the doctor's and laboratory's protocols.
Fasting is generally not required for the 50 g glucose challenge test offered at Wellcare. Follow the preparation instructions provided by the healthcare team for the test; do not eat or engage in strenuous physical activity during the one-hour waiting period. If the screening result is high, the doctor will assess whether a diagnostic test is needed.
What risks can gestational diabetes increase for the mother and baby?
If blood glucose remains above target levels, risks such as high blood pressure during pregnancy or pre-eclampsia, a large baby, shoulder dystocia during delivery and the need for a caesarean section may increase. The newborn may experience low blood glucose and certain other problems. Risks vary depending on the individual and how well blood glucose levels are controlled; these problems do not occur in every pregnancy affected by gestational diabetes.
Poorly controlled pre-existing diabetes may increase the risk of congenital anomalies during early pregnancy, when the organs are developing. This risk should not be assessed in the same way as that associated with gestational diabetes diagnosed later in pregnancy.
How is gestational diabetes managed?
The management plan is tailored to the individual. It usually begins with a balanced diet, pregnancy-appropriate physical activity and home blood glucose monitoring at the times recommended by the doctor. Completely eliminating carbohydrates or trying to lose weight rapidly is not appropriate; the meal plan should meet the needs of both mother and baby.
If these measures do not achieve the targets, medication may be needed. Insulin is the preferred medication for controlling blood glucose during pregnancy. The doctor decides whether medication is needed and which medication to use.
Daily fasting and post-meal blood glucose measurements help guide the treatment plan when monitoring gestational diabetes. HbA1c alone does not replace home blood glucose monitoring or the OGTT.
Is follow-up needed after delivery?
Yes. People diagnosed with gestational diabetes are advised to be reassessed with a 75 g OGTT 4–12 weeks after delivery. Even if the result is normal, the risk of type 2 diabetes may increase in later years, so continued diabetes screening is generally needed every 1–3 years.