Gestational diabetes is one of the more common complications of pregnancy, and one of the more consistently misunderstood. The diagnosis comes from a routine blood test, not from symptoms – most women have no idea anything is different. What follows the diagnosis – monitoring, dietary change, possibly medication – can feel disproportionate to a condition that produces no obvious signs of illness.
The concern is real, though. Persistently elevated blood sugar during pregnancy crosses the placenta and stimulates the baby's pancreas to produce extra insulin, causing the baby to grow larger than normal, with specific patterns of fat deposition that increase risks during delivery and in the newborn period. Managing blood sugar well substantially reduces these risks.
Healthbooq (healthbooq.com/apps/healthbooq-kids) covers pregnancy complications and their management.
What Gestational Diabetes Is
Pregnancy hormones – particularly human placental lactogen, progesterone, and cortisol – cause insulin resistance, meaning the body needs more insulin to keep blood sugar normal. Most pregnant women's pancreas compensates. In gestational diabetes, it cannot fully compensate, and blood sugar rises above normal levels.
GDM is defined by glucose values on an oral glucose tolerance test (OGTT). NICE guideline NG3 sets fasting glucose thresholds of 5.6 mmol/L or above and/or a 2-hour glucose of 7.8 mmol/L or above following a 75g glucose drink. The OGTT is offered to women with risk factors: BMI over 30; a previous baby weighing over 4.5kg; a previous GDM diagnosis; a first-degree relative with type 2 diabetes; or a family origin associated with higher diabetes prevalence (South Asian, Black Caribbean, Middle Eastern).
Prevalence in the UK is rising, reflecting rising rates of obesity and older maternal age at first pregnancy. Approximately 1 in 20 pregnancies is affected.
What the Risks Are
The primary concern with uncontrolled GDM is macrosomia: a baby that grows larger than average, particularly with increased fat deposition around the shoulders, which increases the risk of shoulder dystocia during vaginal delivery. Macrosomic babies also have higher rates of birth injury.
After birth, babies of mothers with GDM are at risk of hypoglycaemia in the first hours of life. The baby's pancreas has been overproducing insulin in response to maternal high blood sugar; when the supply of maternal glucose stops at delivery, the baby can become hypoglycaemic. Newborns are monitored with heel-prick blood glucose checks for 24-48 hours after delivery.
Longer-term, babies born to mothers with GDM have a higher risk of childhood obesity and developing type 2 diabetes later in life. The 2008 study by Dana Dabelea at the University of Colorado found that intrauterine exposure to diabetes is an independent risk factor for metabolic disease in offspring, beyond genetic inheritance.
For the mother, GDM is associated with increased risks of preeclampsia, polyhydramnios (excess amniotic fluid), caesarean birth, and shoulder dystocia.
Monitoring Blood Sugar
Women with GDM monitor their own blood glucose at home using a finger-prick glucometer, typically four times daily: fasting (before breakfast) and 1 or 2 hours after each meal. NICE NG3 target glucose levels are: fasting below 5.3 mmol/L; 1 hour after meals below 7.8 mmol/L (or 2 hours below 6.4 mmol/L if measuring at 2 hours). Women log readings and review them with their diabetes care team.
Dietary Management
Diet is the first-line treatment. The aim is not a low-calorie or very restrictive diet – adequate nutrition in pregnancy remains the priority – but a diet that minimises rapid rises in blood glucose. Carbohydrates are not eliminated but spread evenly across the day and chosen from lower-glycaemic sources: wholegrain bread rather than white, oats rather than sugary cereals, pulses and vegetables rather than large portions of starchy foods at once.
A Diabetes UK-registered dietitian will usually provide individual advice. Three meals and two or three snacks is a common pattern, which prevents both blood sugar spikes from large meals and the hunger that leads to overeating later. Physical activity after meals also blunts the postprandial blood sugar rise.
When Medication Is Needed
Roughly 10-20% of women cannot achieve target glucose levels through diet and activity alone, and medication is added. Metformin is used first in many centres: it improves insulin sensitivity and is taken by mouth. The MiG trial (Rowan et al., NEJM 2008) found metformin as effective as insulin for achieving glycaemic control and associated with lower rates of maternal weight gain and neonatal hypoglycaemia.
Insulin is used when metformin is insufficient or contraindicated. Long-acting insulin is added first (to address fasting levels), then short-acting insulin before meals if needed. Women are trained to self-inject and to adjust doses in response to their glucose readings.
Labour and Delivery
GDM affects labour planning. Women with well-controlled GDM on diet alone are typically offered induction between 40 and 41 weeks, as the risk of stillbirth rises more steeply than in non-diabetic pregnancies in the latter days of term. Those on medication are offered induction at around 38-39 weeks. Continuous fetal monitoring during labour is recommended. A neonatologist or paediatrician will be present at delivery to assess the baby.
Blood sugar is monitored during labour and managed with insulin infusion if it rises.
After Delivery
Blood sugar typically normalises rapidly after delivery and the placenta is delivered. Women stop their diabetes medications immediately. A fasting glucose is checked at 6-13 weeks postpartum to confirm resolution. Approximately 50% of women who had GDM will develop type 2 diabetes within 10-15 years of their pregnancy. Breastfeeding reduces this risk. Annual HbA1c checks are recommended indefinitely.
Key Takeaways
Gestational diabetes (GDM) is a form of high blood sugar that develops during pregnancy and affects approximately 1 in 20 pregnancies in the UK. It is diagnosed through an oral glucose tolerance test (OGTT) between 24 and 28 weeks. Most cases are managed with diet and physical activity; around 10-20% require medication, either metformin or insulin. GDM increases the risk of a large baby, stillbirth, preeclampsia, and caesarean birth, but good blood sugar control significantly reduces these risks. Most women's blood sugar returns to normal after delivery, but GDM carries a 50% lifetime risk of developing type 2 diabetes.
