Can the Glucose Loading Test Harm the Baby? What Do TJOD and ACOG Say?

In antenatal care, one of the tests whose safety is widely debated is the glucose loading test (oral glucose tolerance test, OGTT) ¹. The question expectant mothers ask their doctor — “will it harm the baby?” — reflects this debate. According to a public information article by the Türk Jinekoloji ve Obstetrik Derneği (TJOD, the Turkish Society of Obstetrics and Gynaecology), this test “is an internationally accepted test, and the 50–75 grams of sugar given for the test does not cause any harm to the baby” ¹. Below, we explain step by step the scientific framework behind this answer, how the test is done and what the result means.

Another common worry is the belief that the test will “give the mother diabetes”. TJOD states that this is a mistaken idea: the test only helps to reveal a condition that is already present; if there is no problem with sugar metabolism, the sugary drink does not cause any disturbance in blood sugar ¹. According to the same article, the amount of sugar taken in the test is “not of very significant proportions” ¹. In other words, the test is not a lasting “load” but a brief, controlled measurement; and it is the doctor who follows your pregnancy who evaluates and interprets the result.

What is gestational diabetes, and why does it appear in pregnancy?

Gestational diabetes (commonly called ‘pregnancy diabetes’) is high blood sugar that develops during pregnancy, and it is one of the most common medical problems of pregnancy ². The placenta (the afterbirth) releases hormones to nourish the baby; one of these, placental lactogen (HPL), creates resistance to the action of insulin, which lowers the mother’s blood sugar ¹. In most expectant mothers, the pancreas maintains the balance by producing more insulin; in some pregnant women, however, this increase is not enough, and blood sugar rises, especially in the last three months ¹.

This matters because gestational diabetes that goes unnoticed and unmanaged can lead to the baby being born larger than it should be (macrosomia), difficulties during birth and a higher chance of caesarean section ^1,3. Straight after birth, low blood sugar (hypoglycaemia), breathing problems and jaundice are more common in the baby; in the mother, the likelihood of high blood pressure and pre-eclampsia (‘toxaemia of pregnancy’) increases ³. The test is done to detect this condition in time and so help prevent its possible consequences; the aim is not to frighten, but to plan the rest of the pregnancy.

How is the test done? What do 50, 75 and 100 grams mean?

The method described in the TJOD article, and set out in the Sağlık Bakanlığı (Ministry of Health) guideline for pregnant women outside the risk group, has two steps ^1,4. In the first step, a drink containing 50 grams of glucose is taken and blood sugar is measured one hour later; you do not need to fast for this step ^1,5. If the result is above 130 or 140 mg/dl, depending on the threshold used, the second step follows: after three days of a normal diet without cutting down on carbohydrates, you come in after a 12-hour fast, drink 100 grams of glucose, and four separate blood samples are taken over three hours. The diagnosis is made if two or more of the four values are high ¹.

The second option is the 75 gram, two-hour test; screening and diagnosis are completed in the same session. In its 2024 update, the American College of Obstetricians and Gynecologists (ACOG) recommends the two-step approach at weeks 24–28, and states that the 75 gram test may also be used when considered appropriate for the population ⁵. The Sağlık Bakanlığı (Ministry of Health) guideline, on the other hand, specifies the 75 gram diagnostic test for pregnant women in the risk group, and the 50 gram screening test at weeks 24–28 for other pregnant women ⁴. Which method is chosen is decided together with your doctor.

In which week is it done? Is an early test needed?

The test is recommended for all pregnant women between weeks 24 and 28, whether or not they have risk factors ^1,4,5. The reason for this timing is that insulin resistance caused by placental hormones becomes more marked, particularly in the last three months of pregnancy ¹. TJOD lists women with a family history of diabetes and overweight pregnant women at the top of the risk group, but stresses that screening is carried out for all pregnant women ¹. According to ACOG, gestational diabetes can also develop in women with no risk factors; screening is therefore recommended for all pregnant women, regardless of family history and weight ³.

Because there is no consistent evidence that early diagnosis and treatment benefit mother and baby, ACOG 2024 does not recommend routine screening for gestational diabetes before week 24; in women with risk factors, checking blood sugar at the first visit continues, to detect diabetes that was present before pregnancy ⁵. The Sağlık Bakanlığı (Ministry of Health) guideline takes a different approach: in a pregnant woman in the risk group, blood sugar is checked at the first visit; if there is no overt diabetes, the 75 gram diagnostic test is done, and if this is normal, it is repeated at weeks 24–28; for those outside the risk group, only the 50 gram screening at weeks 24–28 is used ⁴. TJOD also recommends the test at the first appointment for pregnant women with risk factors ¹.

Where does the worry that “it harms the baby” come from?

The worry stems from the idea that the concentrated sugary drink taken all at once will reach the baby. However, TJOD clearly states that the 50 or 75 grams of sugar given has no toxic effect on the baby ¹. This safety statement is made for the 50–75 gram doses; the 100 gram three-hour test is the diagnostic step in ACOG’s standard two-step approach ⁵. According to TJOD, the real risk is not the test itself but gestational diabetes that goes unnoticed; the test only makes an existing condition visible ¹.

The actual discomforts of the test are minor: nausea and vomiting are the most common; a needle is needed to take blood. TJOD’s practical tip is to dissolve the glucose in plenty of water and add a little lemon ¹. There is also a debate about “overdiagnosis”: in the US Preventive Services Task Force (USPSTF) 2021 evidence report, a pooled analysis of five randomised trials including 25 772 pregnant women found that 11.5% were diagnosed with the 75 gram method and 4.9% with the two-step method; however, no improvement was found in health outcomes for mothers and babies ⁶. ACOG 2024 likewise leaves the choice of method to the characteristics of the population and the doctor’s clinical judgement ⁵.

What happens if the result is high?

A diagnosis does not mean that the pregnancy is in danger; it means that the care plan changes. The first step is reorganising your eating pattern and building a habit of regular physical activity; in many expectant mothers, blood sugar is brought into balance with these changes alone ^1,3. If this is not enough, the doctor plans medical treatment; insulin, which is preferred in pregnancy, does not cross the placenta and therefore does not reach the baby ³. Check-ups become more frequent and you are taught to monitor your blood sugar at home; most women with well-controlled gestational diabetes carry their pregnancy to term and can have a vaginal birth ³.

Gestational diabetes usually resolves after the birth; however, monitoring does not end, because the likelihood of developing permanent diabetes later on is increased. ACOG recommends a blood sugar test 4–12 weeks after the birth and, if the result is normal, repeating it every 1–3 years ³. It is also important to tell the baby’s doctor about the mother’s history of gestational diabetes; these children should be monitored for weight and blood sugar as they grow ³. In short, the glucose loading test is a checkpoint that helps protect both the current pregnancy and the mother’s long-term health.

If you have symptoms, please consult an obstetrics and gynaecology specialist.