Glycemic test for gestational diabetes

During pregnancy, a GCT glycemic curve test is done for the diagnosis of gestational diabetes. Where appropriate, OGTT oral glucose loading, where appropriate. The criteria currently in use for screening and diagnosing GDM are not unambiguous. According to the new Guidelines for the screening and diagnosis of gestational diabetes from The HAPO Study Cooperative Research Group, the two-step procedure (“glucose miniload” (GCT) + OGTT in cases with positive miniload) previously used is considered outdated and is therefore not recommended.


To screen and diagnose gestational diabetes, or to identify the presence of alterations in carbohydrate metabolism, during pregnancy glucose curve tests (GCT) and oral glucose loading tests (OGTT) are generally done.

These two tests can be done in combination, although most of the time only the glycemic curve test (GCT) is sufficient. The glycemic curve test is usually performed between the 24th week and the 28th week of pregnancy.

However, it can also be done from the first prenatal visit in the case of a woman,

  • obese or with a family history of diabetes,
  • with a personal history of gestational diabetes,
  • 35 years of age or older,
  • with fetal macrosomia (the fetus at birth weighs more than 4 kg) in previous pregnancies;
  • of high-risk ethnicity,
  • and other factors,
  • So, in all those cases of women with a high risk of gestational diabetes.

Let’s see these two tests separately.

The Glucose Challenge Test (GCT))

Why it’s done

During pregnancy the glycemic curve test is used to screen for gestational diabetes, which is an alteration of glucose metabolism that develops in the second or third trimester of pregnancy in 18% of women. This type of diabetes occurs only during pregnancy, and should not be confused with prepregnancy diabetes (the one already present before pregnancy). However, this is a phenomenon that does not normally involve serious dangers.

Abnormal results from these tests — done early in pregnancy — rather than gestational diabetes could indicate pre-existing type 2 diabetes not previously found.

Most women with gestational diabetes give birth to healthy babies. However if this is not carefully managed it can lead to various pregnancy complications such as, for example,

  • preeclampsia or excessive fetal growth, which could increase the risk of birth injury or require a cesarean delivery,
  • hypoglycemia (low blood glucose levels) soon after birth,
  • respiratory problems and increased risks of obesity and type 2 diabetes in adulthood.

The examination of the glycemic curve in pregnancy is performed at two different times:

  1. Once her basal blood glucose (the concentration of glucose in fasting plasma) has been detected, in order to measure her glycemic curve, a sugar solution is given to the pregnant woman.
  2. An hour later, the level of sugar in her blood is measured.

If the results of this first test are abnormal and show high blood sugar levels, the expectant mother will be prescribed a glucose tolerance test (OGTT), which takes about three hours.

in preparation for the GCT test

The day before taking the glycemic curve test (GCT) the woman can eat and drink normally.

how the GCT test is done

The pregnancy glycemic curve test (GCT) takes place temporally as follows:

  1. an fasting blood sample is taken in order to measure the basal blood glucose of the expectant mother,
  2. then she is given a solution containing a predefined amount of glucose;
  3. in the next hour, the expectant mother must remain seated, possibly relaxed, without smoking or eating;
  4. After 60 minutes from the first sip, a second blood sample is taken.

the possible results of the GCT test

Pregnancy glycemic curve test results are given in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L).

  1. A blood sugar level below 140 mg/dL (7.8 mmol/L) is considered normal.
  2. Instead, a blood sugar level between 140 mg / dL (7.8 mmol / L) and 180 mg / dL could indicate the existence of gestational diabetes: in this case the doctor – in order to make a more precise diagnosis, will carry out another test, usually the oral glucose load test (OGTC).
  3. To confirm the diagnosis, however, it is not necessary to use the OGTT if the pregnancy glucose curve test (GCT) shows a value greater than 180 mg / dl.
  4. In the case of gestational diabetes it is necessary for the woman to undergo more frequent ultrasound checks.

After taking the glycemic curve test (GCT), the expectant mother can spend the rest of the day normally.

The Oral Glucose Tolerance Test (OGTT) test (OGTT )

Why it is done in pregnancy

Usually the oral glucose load test (OGTT) is done – with due precautions – when the pregnancy glycemic curve test (GCT) indicates blood glucose levels high enough to suspect the presence of gestational diabetes.

in preparation for the OGTT test

It is necessary that the pregnant woman has been on an absolute fast (only water is allowed) for at least 8 hours – but without exceeding 14 hours.

In the days preceding the OGTT test, the woman must follow a normally balanced diet, without caloric excesses or particular deprivations (obviously alcohol must be avoided); An intake of at least 150 grams of carbohydrates per day is generally recommended.

It is necessary that in the two or three days before the test, the expectant mother avoids intense physical exertion. It is also very important the preventive suspension of any drugs interfering with glucose metabolism according to the advice of your doctor: this must be warned in time for the examination (at least a few weeks).

how the OGTT test is done

The exam usually takes place in the morning. The OGTT glucose tolerance test for gestational diabetes detection is performed in several stages.

  • A nurse takes a blood sample from a vein in the pregnant woman’s arm to check the fasting sugar concentration (basal blood sugar).
  • Then he gives her 100g of glucose dissolved in 250-300ml of water.
  • In the following hours, the expectant mother must remain seated, without smoking or eating, possibly relaxed (stress can distort the results).
  • Your blood glucose is then measured at regular intervals, 60, 120 and 180 minutes after ingesting the first sip of glucose solution.

After taking the glycemic curve test, the expectant mother can spend the rest of the day normally.

possible results of OGTT tests

If two or more glycemic values are found higher than the reference ones, you have a diagnosis of gestational diabetes. If it is found that only one value is higher, there is the diagnosis of glucose intolerance in pregnancy.

The normal (reference) limits of the results of this examination are,

  • fasting: less than 95 mg/dL or 5.2 mmol/L
  • at 60 minutes: less than 180 mg/dL or 10.0 mmol/L
  • at 120 minutes: less than 155 mg/dL or 8.6 mmol/L
  • at 180 minutes: less than 140 mg/dL or 7.7 mmol/L

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

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