Gestational diabetes

Gestational diabetes (GDM) — pregnancy sugar

Gestational diabetes mellitus is diabetes first recognised in pregnancy — not pre-existing diabetes (different pathway).

~5% UK pregnancies — rising with maternal obesity and age.

Placenta produces human placental lactogen and other hormones → physiological insulin resistance — GDM when pancreas cannot keep up.

Who gets screened

All pregnant women — risk stratification:

Early OGTT (booking) if:

• previous GDM
• BMI ≥30 (or ≥27.5 South Asian)
• previous macrosomic baby ≥4.5kg
• first-degree relative diabetes
• PCOS
• high-risk ethnicity

Universal OGTT 24–28 weeks if not already diagnosed

Diagnosis thresholds (NICE)

75g OGTT:

• fasting ≥5.6 mmol/L, OR
• 2-hour ≥7.8 mmol/L

One abnormal value — GDM

Why it matters

Maternal:

• pre-eclampsia risk
• polyhydramnios
• caesarean — large baby
• future type 2 diabetes

Fetal/neonatal:

• macrosomia
• shoulder dystocia — obstetric emergency
• neonatal hypoglycaemia — baby’s insulin high when cord cut
• respiratory distress
• stillbirth — rare with care

Good control — outcomes near normal.

Management

Blood glucose monitoring

Capillary testing — fasting and 1-hour post-meal (or 2-hour per local protocol)

Targets (NICE):

• fasting <5.3 mmol/L
• 1-hour post-meal <7.8 or 2-hour <6.4

Lifestyle

• carbohydrate consistency — don’t eliminate — baby needs glucose
• walk after meals
• dietitian — specialist antenatal diabetes clinic

Medication if targets missed

Metformin — crosses placenta minimally — NICE option

Insulin — gold standard if metformin insufficient or contraindicated

Glibenclamide — selected centres — less first-line

Obstetric care

• growth scans
• induction timing — often 38–40 weeks if on insulin/macrosomia — individual
• hospital birth if insulin — protocol varies

After birth

Placenta gone — insulin resistance drops — stop diabetes meds unless told otherwise

Fasting glucose 6–13 weeks — OGTT if borderline

If normal:

• annual HbA1c or glucose — lifelong
• lifestyle — halve type 2 risk

Breastfeeding — encouraged

GDM vs type 2 in pregnancy

Pre-existing diabetes — higher baseline risk — preconception HbA1c optimisation — retinopathy check

See type 2 diabetes and PCOS — shared risk.

GDM is common, manageable, temporary — missed postnatal test misses lifetime diabetes warning.

Common questions

What causes gestational diabetes?

Placental hormones block insulin action — insulin resistance rises in pregnancy. If pancreas cannot produce enough extra insulin, blood sugar rises. Risk factors — BMI over 30, previous gestational diabetes, family history type 2, polycystic ovary syndrome, South Asian/Black/Caribbean/Middle Eastern ethnicity, previous large baby over 4.5kg.

How is gestational diabetes tested?

Oral glucose tolerance test — fasting blood sample, drink 75g glucose drink, second sample 2 hours later. Diagnosed if fasting ≥5.6 mmol/L or 2-hour ≥7.8 mmol/L (NICE thresholds). Earlier testing if previous GDM or glycosuria.

Does gestational diabetes harm the baby?

Poorly controlled glucose crosses placenta — baby grows large (macrosomia), increases birth injury and shoulder dystocia risk, neonatal low blood sugar after delivery, jaundice, stillbirth risk elevated though absolute risk low with good care. Good control minimises complications.

How is gestational diabetes treated?

Blood glucose monitoring — fasting and post-meal targets. Diet — spaced carbohydrates, portion control. Exercise if safe in pregnancy. Metformin or insulin if lifestyle insufficient — both safe in pregnancy when indicated. Team includes obstetrician, diabetes midwife, dietitian.

Does gestational diabetes go away after birth?

Usually resolves immediately or within days after placenta delivered. Fasting glucose test 6 to 13 weeks postpartum — if normal, annual diabetes screening recommended lifelong due to high type 2 risk. Breastfeeding may modestly reduce later diabetes risk.

Sources

• NHS — Gestational diabetes
• NICE — Diabetes in pregnancy