Gestational Diabetes — Screening, Blood Sugar Targets, Diet & Insulin Guide — Symptoms, Causes & Treatment | MyMedicPlus

Gestational diabetes mellitus (GDM) is defined as glucose intolerance first diagnosed during pregnancy (typically in the second or third trimester), that was not overtly diabetic before pregnancy. It is the most common metabolic complication of pregnancy, affecting approximately 14% of pregnancies globally (IDF 2023 estimates), with prevalence rates ranging from 10% to 28% depending on population ethnicity and diagnostic criteria. GDM arises from insufficient insulin secretory capacity to overcome the physiological insulin resistance of pregnancy — created by placental hormones (human placental lactogen, cortisol, oestrogen, and progesterone) that peak in the second and third trimesters. It resolves in most women after delivery — but leaves a major legacy of risk: a 7-fold increased lifetime risk of Type 2 diabetes in the mother, and significant fetal programming effects increasing the child's risk of obesity and T2DM. Without treatment, GDM significantly increases risks of macrosomia (large baby), birth trauma, caesarean delivery, neonatal hypoglycaemia, stillbirth, and pre-eclampsia.

Physiological insulin resistance increases progressively during normal pregnancy due to placental hormones (HPL, cortisol, oestrogen, progesterone, and TNF-alpha) — a mechanism designed to ensure adequate glucose supply to the fetus. In most pregnancies, maternal pancreatic beta cells compensate by increasing insulin secretion 2-3 fold. GDM develops when this beta-cell reserve is insufficient — revealing underlying insulin secretory impairment. Risk factors: previous GDM (50-70% recurrence risk); BMI above 30 kg/m2 (the strongest modifiable risk factor); family history of T2DM (first-degree relative); previous macrosomic baby (birth weight above 4.5 kg); South Asian, Black, Middle Eastern, or Pacific Islander ethnicity (2-4 times higher risk than White European); polycystic ovary syndrome (PCOS — pre-existing insulin resistance); maternal age above 35; multiple pregnancy; and unexplained previous stillbirth. Obesity-driven GDM is increasing globally as maternal obesity rates rise — GDM prevalence has doubled in 20 years in some populations.

GDM is typically asymptomatic — most cases are identified only through routine screening. Occasionally, women may notice classic hyperglycaemia symptoms — polydipsia (increased thirst), polyuria (increased urination), and fatigue — but these overlap with normal pregnancy symptoms, making clinical diagnosis unreliable. Glycosuria (glucose in urine on dipstick) may prompt investigation but has poor sensitivity and specificity for GDM. Clinical clues on serial ultrasound assessment: fetal macrosomia (abdominal circumference above the 90th centile for gestation); polyhydramnios (excess amniotic fluid — glucose acts as an osmotic agent increasing fetal urinary output); unexplained elevated symphysis-fundal height. Screening is therefore universal in high-risk groups and widely recommended universally in many countries to detect GDM before complications develop.

75g oral glucose tolerance test (OGTT): the diagnostic gold standard for GDM. Timing: at 24-28 weeks of gestation in women with risk factors (in the UK — NICE NG3); at the first antenatal visit in women with very high risk (previous GDM, BMI above 30, glycosuria, or previous macrosomic baby above 4.5 kg) — earlier testing at 16 weeks if previous GDM. Diagnostic criteria (NICE 2015): fasting plasma glucose above 5.6 mmol/L OR 2-hour post-glucose above 7.8 mmol/L. WHO/IADPSG criteria (used in many countries): fasting above 5.1 mmol/L OR 1-hour above 10.0 mmol/L OR 2-hour above 8.5 mmol/L. HbA1c: used at booking in women with risk factors — HbA1c above 48 mmol/mol (6.5%) at booking indicates pre-existing Type 2 diabetes (not GDM) and requires immediate treatment. HbA1c is NOT used to screen for or diagnose GDM after 24 weeks as red cell turnover alters in pregnancy, making it unreliable. Self-monitored blood glucose (SMBG): all women diagnosed with GDM should test fasting and 1-hour post-meal glucose 4 times daily using a home glucometer. Continuous glucose monitoring (CGM — FreeStyle Libre): increasingly used in insulin-treated GDM.

Treatment targets (NICE NG3): fasting blood glucose below 5.3 mmol/L; 1-hour post-meal below 7.8 mmol/L. Medical nutrition therapy (MNT) is the foundation of GDM management: individualised dietary counselling from a specialist diabetes dietitian; carbohydrate controlled diet (reducing and spreading carbohydrate intake over meals, avoiding refined sugars and high-glycaemic-index foods); total carbohydrate typically 40-45% of total calories, prioritising low-GI sources (basmati rice, granary bread, lentils, pasta al dente); small, frequent meals (3 main meals and 2-3 snacks). Walking and moderate physical activity after meals significantly reduces post-meal glucose spikes and can defer or prevent insulin therapy. Pharmacological therapy (if glucose targets not achieved within 1-2 weeks of diet): Insulin: first-line pharmacotherapy for GDM in NICE guidelines — rapid-acting insulin analogues (insulin aspart/lispro) before meals for post-meal control; isophane (NPH) insulin for persistent fasting hyperglycaemia; insulin requirements increase progressively through pregnancy. Metformin: used as adjunct or alternative to insulin in some guidelines — reduces insulin dose requirements; crosses the placenta (long-term neonatal safety data reassuring to date); not recommended as first-line in NICE NG3. Monitoring and obstetric care: growth ultrasound at 28, 32, and 36 weeks; umbilical artery Doppler for macrosomic babies; fetal kick counting. Postpartum: all women with GDM should have fasting plasma glucose or HbA1c at 6-13 weeks postpartum to exclude persistent T2DM.

Gestational diabetes carries significant risks for both mother and baby if inadequately controlled. Fetal and neonatal complications: macrosomia (birth weight above 4 kg — affects 30–35% of untreated GDM) increases risk of shoulder dystocia, brachial plexus injury, and emergency caesarean section; neonatal hypoglycaemia (fetal hyperinsulinaemia from maternal hyperglycaemia causes neonatal glucose to drop dangerously after birth — requires glucose gel or IV glucose, and NICU admission); polycythaemia, jaundice, and respiratory distress syndrome are more common in macrosomic neonates; stillbirth risk is elevated in poorly controlled GDM particularly beyond 38 weeks. Maternal complications: pre-eclampsia is 2–4 times more common in GDM; operative delivery (caesarean section, instrumental delivery) rates are substantially higher; maternal hypoglycaemia from insulin therapy carries acute risks; genital infections are more frequent with hyperglycaemia. Long-term maternal risk: GDM is a major risk factor for type 2 diabetes — 50% of women with GDM develop T2DM within 5 years and 70% within 10 years; early postpartum screening and diabetes prevention programme referral are essential to mitigate this risk. Long-term child risk: offspring of GDM mothers have a significantly higher risk of childhood obesity, metabolic syndrome, and type 2 diabetes in adolescence and adulthood — the intrauterine hyperglycaemic environment programmes adipogenesis and insulin secretory capacity, creating a transgenerational metabolic burden.

Pre-conception weight loss in obese women is the most effective prevention strategy — achieving a BMI below 30 before pregnancy reduces GDM risk by 40-60%. The landmark DALI and LIMIT trials showed that structured diet and exercise interventions in overweight/obese pregnant women reduced GDM incidence. Exercise during pregnancy: 30 minutes of moderate-intensity aerobic activity most days of the week (150 minutes/week) is safe and recommended from 12 weeks onwards; post-meal walking of 10-15 minutes is particularly effective at reducing post-meal glucose spikes. Low-glycaemic-index diet: adopting a low-GI diet before and during pregnancy reduces GDM risk. For women with previous GDM: structured lifestyle programme referral immediately after delivery; repeat OGTT in subsequent pregnancy at 16 weeks and 24-28 weeks. Long-term diabetes prevention: all women with previous GDM should receive structured diabetes prevention programme referral (NHS DPP in the UK) — intensive lifestyle intervention reduces T2DM progression by 58% (DPP trial), most effective in this high-risk group.

All pregnant women should inform their midwife or GP of GDM risk factors at the first antenatal booking appointment (before 12 weeks), so that OGTT screening can be arranged at 24-28 weeks or earlier if indicated. Contact your midwife or diabetic team urgently if, after GDM diagnosis, you experience: blood glucose readings consistently above targets despite dietary changes (indicating need for pharmacotherapy promptly — delays increase fetal macrosomia risk); any episode of hypoglycaemia (blood glucose below 4 mmol/L — particularly important if on insulin or metformin); absent or reduced fetal movement; sudden increase in thirst and urination; or any signs of pre-eclampsia (headache, visual disturbance, swelling of the face or hands, epigastric pain). Women with GDM should be seen in a specialist joint antenatal-diabetes clinic. Postpartum follow-up with a fasting glucose or HbA1c at 6-13 weeks is essential and must not be missed.