Glucose tolerance test in pregnancy (3 points)

Glucose tolerance testing in pregnancy (pregnancy OGTT) detects gestational diabetes, high blood sugar first appearing during pregnancy. Glucose is measured in three blood samples: fasting, one hour and two hours after drinking a solution containing 75 g glucose. Testing is performed between weeks 24 and 28 of pregnancy.

Who is this test for

• all pregnant women between weeks 24 and 28 who have not previously been diagnosed with diabetes or gestational diabetes;
• pregnant women without symptoms or risk factors, because gestational diabetes usually causes no symptoms;
• pregnant women with excess body weight, for whom the doctor may also recommend earlier testing;
• pregnant women with gestational diabetes in a previous pregnancy, for whom the doctor may also recommend earlier testing.

What gestational diabetes is and why testing is performed

Placental hormones in the second half of pregnancy reduce the body’s sensitivity to insulin. If the pancreas cannot secrete enough additional insulin, blood glucose rises. Gestational diabetes usually causes no symptoms, so it is detected only through testing.

Untreated, it increases the risk of an excessively large baby and birth complications, low glucose in the newborn and pre-eclampsia. Once detected, dietary adjustment, regular sugar measurements and insulin if necessary substantially reduce these risks. It usually resolves after childbirth, but women who have had it have a higher risk of type 2 diabetes later in life.

According to IADPSG and World Health Organization recommendations (WHO 2013), also used in Croatia, testing is performed for all pregnant women without previously diagnosed diabetes or gestational diabetes, rather than only those with risk factors. Fasting glucose is usually measured at the first antenatal visit to detect previously unrecognised diabetes or abnormal blood sugar. The doctor determines how this result is interpreted in early pregnancy and whether a later tolerance test is needed according to pregnancy week and the applicable protocol. Always agree whether and when to have testing with your gynaecologist.

Preparation for the test

• Eat normally for three days before testing, with enough carbohydrates (bread, pasta, rice, potatoes, fruit). Do not start a diet immediately before testing.
• Fast for 8 to 14 hours before testing. You may drink plain water.
• Do not smoke or chew gum that morning.
• Do not stop medicines or prenatal vitamins yourself. Tell your doctor what you take, because some medicines, such as corticosteroids, raise glucose.
• If you have an acute illness, such as an infection with fever, or severe vomiting, postpone testing until you recover.

How the test works

1. The first blood sample is taken from a vein in the morning after fasting.
2. Within five minutes, drink a solution of 75 g glucose in around 3 dL water.
3. Spend the next two hours in the laboratory: sit and rest, without eating, drinking or smoking.
4. The second and third blood samples are taken one hour and two hours after you started drinking the solution.

The solution is very sweet and causes nausea in some pregnant women. If you vomit, testing is stopped and repeated another day. Bring something to eat afterwards.

How results are interpreted

Different thresholds apply in pregnancy than in other adults (IADPSG, WHO 2013). Gestational diabetes is diagnosed if at least one value equals or exceeds the threshold:

Sample	Gestational diabetes threshold
fasting	5,1 mmol/L
after 1 hour	10,0 mmol/L
after 2 hours	8,5 mmol/L

If fasting glucose is 7,0 mmol/L or above, or the two-hour value is 11,1 mmol/L or above, this is diabetes in pregnancy requiring more urgent investigation. The thresholds for non-pregnant adults (7,8 and 11,1 mmol/L after two hours) are described under two-point glucose tolerance testing and do not apply during pregnancy.

The gynaecologist managing your pregnancy interprets the results. An elevated result usually leads to referral to a diabetologist, who advises diet and self-monitoring and starts treatment if necessary. If you have had gestational diabetes, a two-point glucose tolerance test is recommended 4 to 12 weeks after childbirth, followed by regular sugar checks.

How to arrange testing

You can come for blood collection without an appointment or referral at one of our branches throughout Croatia (listed on the Contact). Attend in the morning after fasting. Prices for all tests and panels are in the service price list.

Frequently asked questions

Is OGTT mandatory during pregnancy?

Testing is recommended for all pregnant women between weeks 24 and 28 because gestational diabetes usually causes no symptoms. The decision belongs to you and your gynaecologist; if you do not want testing, discuss other monitoring methods.

Can the glucose solution harm the baby?

No. The amount of glucose is similar to that in a sweetened drink or carbohydrate-rich meal and does not harm the baby. It may cause temporary nausea or dizziness in the mother; tell the staff immediately if this occurs.

Can I drink water before testing?

While fasting, you may drink plain water. Avoid coffee, tea, juice and milk. After drinking the solution, do not drink anything until the final blood collection.

What if only one value is elevated?

One value equal to or above the threshold is sufficient to diagnose gestational diabetes. In most pregnant women it is well controlled through diet and activity, while the gynaecologist and diabetologist arrange further monitoring.