Gestational diabetes (GDM)

GDM is a condition where diabetes develops in pregnancy. The cause is unknown. This can be due to hormonal fluctuations in pregnancy, where your body fails to make adequate insulin to overcome the anti-insulin effect of your placental hormones. Insulin is a hormone that helps to maintain normal blood sugar levels. This condition is temporary.

It is common. About 4 -5 per 100 pregnant mums can develop this. GDM is usually seen in mid and late pregnancy.

Increased risk of GDM may be associated with:

  • BMI exceeding 25 or 30 (23 for Asians). (Body mass index).
  • History of a sedentary lifestyle.
  • Family history of diabetes in a 1st-degree relative.
  • Ethnicity linked to higher risk (African, Afro-Caribbean, Middle Eastern, South East Asian, Chinese, or other high-risk ethnicities).
  • Previous pregnancy affected by GDM.
  • History of delivering a big baby (≥ 4000 g).
  • High blood pressure (140/90 mm Hg or more).
  • Low HDL (high-density) cholesterol levels (≤ 35 mg/dl or 0.90 mmol/L). (HDL is the good cholesterol).
  • Elevated fasting triglyceride levels (≥ 250 mg/dL or 2.82 mmol/L). (High levels of fasting fat in the blood).
  • Diagnosis of polycystic ovarian syndrome (PCOS). (A disease where many cysts are noted in both ovaries with other hormonal imbalances).
  • Presence of conditions associated with insulin hormone resistance (e.g., acanthosis nigricans, morbid obesity).
  • Hb A1C level ≥ 5.7% (if tested before pregnancy). (A blood chemical that indicates your average blood sugar levels over the past two to three months).
  • Cardiovascular disease. (Heart Disease)

Most women with GDM experience no symptoms in pregnancy. Sometimes if blood sugar is very high antenatally, symptoms of dry mouth, tiredness, genital area itching, thrush, thirst and so on, can be experienced. The UK adopts a risk-based screening strategy. If there is any one of the above risk factors at the booking antenatal visit, a blood test to rule out GDM is required. This is usually a 75-g 2-hour oral glucose tolerance test (OGTT), done between 24 to 28 weeks of pregnancy. The procedure details will be explained if needed. Blood samples are tested before drinking a sugary drink (fasting sample) and two hours after.

If fasting blood sugar levels are 5.6 mmol/litre or above, or a 2-hour blood sugar level is 7.8 mmol/litre or above, GDM is diagnosed.

  • Sometimes, if there is a history of previous GDM, this test might be required before 24 weeks. If this is normal, it may be repeated again between 24-28 weeks. Additionally, urine is routinely checked for glucose during all antenatal visits.
  • People with GDM are looked after by our specialist multidisciplinary diabetic team. They may need more frequent hospital antenatal visits. We will demonstrate and educate mum on how to monitor blood glucose at home, several times a day.
  • They will need to keep a record of their readings. If these readings are very low (hypoglycemia) or very high, they will need to contact us. We will give clear instructions on what will be needed next.
  • They may need several ultrasound scans to check that the baby/ies are growing well.

Most women with GDM have healthy pregnancies. But if blood sugar levels are high and not well controlled, the risks may include:

  • A need to induce childbirth and a risk of Caesarean birth. (Caesarean births are seen in 25% of women with GDM who are on Medicines and 17% in diet-controlled GDM: ACOG)
  • High blood pressure and/or pre-eclampsia (PE) in pregnancy. (PE is seen in 9.8% of people, with a fasting glucose less than 115 mg/dL and 18% people, with a fasting glucose greater than or equal to 115 mg/dL: ACOG)
  • Premature birth.
  • A large amount of fluid around your baby.
  • A large baby weighing over 4 kgs or 9lbs.
  • Complications during the birth of a big baby as “shoulder dystocia”. This is where the large baby’s shoulders are stuck inside the birth passage making it difficult to extricate. There is a risk of nerve injury and fractures to the baby, during vaginal birth.
  • Tears in the birth passage during normal childbirth.
  • More than average bleeding after the birth of the baby/ies.
  • Long-term risks of developing Type 2 diabetes in mum. (70% of women with GDM will develop Type 2 diabetes within 22–28 years after their pregnancy: ACOG) This is more common in obese mums within certain ethnic groups.

The risks baby’s may face include:

  • Low and/or unstable blood sugars at birth.
  • Risk of death, near or during childbirth: stillborn.
  • A higher chance of admission to the Neonatal baby unit. (NICU)
  • A higher chance of jaundice at birth.
  • A large or smaller than normal sized baby.
  • Long-term health risks as obesity and Type 2 diabetes.

What are the target blood sugars?

To reduce the risk of having a large baby, the target blood sugar levels are:

  • 5.3 mmol/litre at fasting,
  • 7.8 mmol/litre 1 hour after meals, or 6.4 mmol/litre 2 hours after meals.
  • We may need to review blood sugars weekly for better blood sugar control.

Treatment decreases any risk of pre-eclampsia in pregnancy and the risk of having a big baby.

Diet and Exercise: 70% of cases of GDM respond to diet and exercises. This works best if the fasting blood sugar was below 7 mmol/litre at diagnosis. We offer:

  • A nutrition assessment and plan after counseling by our registered dietitian/nutrition team.
  • We suggest complex carbohydrates, 3 meals and 2/3 snacks to reduce any blood glucose alterations. (Carbohydrates: 33–40% of the total calories, and the rest of the calories from protein (20%) and fat (40%))
  • We suggest 30 minutes of moderate-intensity aerobic exercise at least 5 days a week or a minimum of 150 minutes per week.
  • It is useful to consider a mixture of aerobic (such as walking) and strength training exercises (such as lifting light weights). Aim for at least 20 minutes per day.
  • We can discuss safe ways to exercise in pregnancy, at OBSGYN, London.
  • GDM is known to improve with walking, at least 10 to 15 minutes or better if 30 minutes, after each meal.
  • We recommend checking blood sugars every day. We will discuss the frequency of these tests.

Insulin and drugs: 30% of mums may need insulin during pregnancy. This is offered if diet and exercise have failed to control blood sugars after a week or two. This is also offered if fasting blood sugars are 7.0 mmol/litre or above at diagnosis. Please continue with diet and exercise, in addition.

  • If blood sugars are between 6.0 and 6.9 mmol/litre and there are added complications as a big baby or a large amount of fluid around baby, it is best to start with insulin and or Metformin, in addition to diet and exercises. We will explain things clearly depending on individual circumstances.
  • The starting insulin dose is low and usually divided. A long-acting or intermediate-acting insulin with a short-acting insulin is normally preferred.
  • Personal plans are made based on your blood sugar control.
  • Short-acting insulin (e.g. insulin lispro and aspart) are recommended, rather than regular insulin, as it has a more rapid action.
  • If there is difficulty in using insulin or any objections to the use of insulin, the oral tablet Metformin can be used.
  • Counseling about Metformin risks including placental crossing over, is offered. However, no long-term studies of its effects on baby/ies are available.
  • Metformin may be associated with premature birth.
  • The initial dose: is 500 mg at night, for 1 week, and then increased to 500 twice a day.
  • We may need baseline blood tests to check creatinine levels. (A chemical released by your kidneys).
  • The side effects of Metformin are belly pain and diarrhea – it is recommended therefore, to take these tablets with meals.
  • The maximum dose that can be used is 2,500 to 3,000 mg a day, in two or three divided doses.
  • We may offer Aspirin 150mg, from 12 weeks to decrease PE risks.
  • If GDM is well controlled on diet: we can consider childbirth at 39w0d to 40w6d. Sometimes we can continue with pregnancy till 40w6d if there are normal antenatal blood sugar tests.
  • If the baby/ies have not been born by this time, a Caesarean section or induction may be offered.
  • If GDM is well controlled on medicines: we may consider a childbirth date at around 39w0d to 39w6d of pregnancy.
  • If GDM is badly controlled: there is no exact timing. We will consider any individual situation before deciding on a date and time of childbirth.
  • Usually birth between 37w0d and 38w6d may be needed.
  • Sometimes birth between 34w0d and 36w6d weeks 0 days are needed for (1) very poor in-hospital blood sugar control or (2) abnormal ultrasound findings of the baby/ies. Our team will discuss options if this is needed.
  • If the fetal weight on ultrasound scan is ≥4500 kgs or 10 lbs: We will offer counseling of the risks vs benefits of a scheduled cesarean section.
  • The anesthetic team will be consulted between 34-36 weeks if there are additional pregnancy risk factors such as obesity.
  • We will monitor blood sugars at hourly intervals during natural childbirth.
  • If these readings are too high (not between 4 mmol/litre and 7 mmol/litre), we may start a variable rate intravenous insulin (VRII) infusion, in active labor, to stabilize blood sugars.
  • We may also start other intravenous fluids.
  • For people on VRII a test for blood potassium levels, every 12 hours, is required. This will help us to adjust the fluids.
  • The infusion is stopped after childbirth.
  • During a Caesarean section, we will monitor blood sugars hourly on the day. An insulin drip on the day of surgery may be necessary. This is until the ability to eat and drink normally is restored.
  • Breastfeeding is safe in GDM. We advise feeds immediately after birth and then every 2-3 hours.
  • We advise expressing and storing colostrum with us, from 36 weeks. This can be used as a supplement to feed baby after childbirth.
  • Baby can stay with mum after birth except in certain situations. We will test baby’s blood sugar levels after birth (at least 3 times) to make sure these are not too low. A normal blood sugar level for a newborn baby is usually 2.0 mmols/L or above.
  • If baby’s blood sugars are low, we will explain the next steps.
  • After childbirth we will stop all diabetic medications and insulin.
  • Before being discharged home we will check mum and baby’s blood sugar levels. We will also make sure the baby/ies are stable and feeding well.
  • We will explain carefully the symptoms that can be experienced if blood sugars are too high in mum.
  • Screening mum between 6 to 13 weeks after childbirth for prediabetes or type 2 diabetes, is offered. This is usually a fasting blood sugar test.
  • If mum fails to have the test above, before 13 weeks we can still offer a fasting blood sugar test or an HbA1c test (a test to check another blood chemical) if a fasting blood sugar test is not possible.
  • If fasting sugars are below 6.0 mmol/litre and/or HbA1c level below 39 mmol/mol (5.7%), there is a low future diabetes risk. However, screening tests for Type 2 diabetes every year, are offered.
  • If fasting sugars are over 6.0 mmol/litre and/or HbA1c level above 39 mmol/mol (5.7%), counseling is offered regarding the need for and type of further tests and the risks of developing Type 2 diabetes.
  • We can offer a referral to the NHS Diabetes Prevention Programme if eligible for this.
  • We will provide detailed advice about lifestyle, diet, exercise and weight management, to decrease all chances of type 2 diabetes, in the future.
  • A healthy lifestyle significantly decreases the chances of having GDM, next time.
  • The overall chance of having GDM again is around 1:3.
  • Please contact us immediately when pregnant again.

If there are any further questions about this leaflet, please contact us at OBSGYN LONDON.

  1. SOGC: Guideline No. 393-Diabetes in Pregnancy: 2019

  2. Diabetes in pregnancy: management from preconception to the postnatal period NICE guideline [NG3]Published: 25 February 2015 Last updated: 16 December 2020

  3. RCOG patient information leaflet: September 2021. This can be accessed here:

  4. NHS patient information leaflet: Gestational Diabetes (2024) This can be accessed here:

  5. ACOG Gestational Diabetes Mellitus Practice Bulletin PB Number 190 February 2018. This can be accessed here: