The number that diagnosed you depends on where you are

This matters more than most pages admit, because the same blood result can be a diagnosis in one country and a normal test in another. There is no single global cut-off, and pretending otherwise is how people end up confused by conflicting advice online.

NICE (UK), guideline NG3, recommendation 1.2.8: diagnose gestational diabetes if the woman has either "a fasting plasma glucose level of 5.6 mmol/litre or above" or "a 2-hour plasma glucose level of 7.8 mmol/litre or above" on a 75 g oral glucose tolerance test.

WHO (2013): gestational diabetes "should be diagnosed at any time in pregnancy if one or more of the following criteria are met: fasting plasma glucose 5.1-6.9 mmol/l (92-125 mg/dl); 1-hour plasma glucose 10.0 mmol/l (180 mg/dl) or above following a 75 g oral glucose load; 2-hour plasma glucose 8.5-11.0 mmol/l (153-199 mg/dl) following a 75 g oral glucose load." WHO grades the quality of that evidence as "very low" and the strength of the recommendation as "weak" in the same document, which is worth knowing.

ACOG (US): WHO's own comparison table records ACOG's approach as a 100 g oral glucose tolerance test with thresholds of fasting 5.3, 1-hour 10.0, 2-hour 8.6 and 3-hour 7.8 mmol/litre, with two or more values required for diagnosis. That is a two-step method, and it needs two abnormal results rather than one.

So a fasting result of 5.2 mmol/litre is not gestational diabetes under NICE and is gestational diabetes under WHO. Your care follows whichever body your maternity service uses. Do not try to average them.

Why you were offered the test at all

NICE does not screen everyone. Recommendation 1.2.2 says to check at the booking appointment for these risk factors: "BMI of 30 kg/m2 or over; previous macrosomic baby weighing 4.5 kg or more; previous gestational diabetes; family history of diabetes (first-degree relative with diabetes); an ethnicity with a high prevalence of diabetes." Anyone with one of those is offered a 75 g two-hour OGTT at 24 to 28 weeks.

If you had gestational diabetes before, recommendation 1.2.6 is different: early self-monitoring of blood glucose, or an OGTT "as soon as possible after booking", and a further OGTT at 24 to 28 weeks if the first is normal. Glycosuria on a dipstick also triggers further testing, at "2+ or above on 1 occasion" or "1+ or above on 2 or more occasions".

What happens in the first week

NICE recommendation 1.2.9 says to offer "a review with the joint diabetes and antenatal clinic within 1 week" and to tell your primary healthcare team. Recommendation 1.2.16 says to refer all women with gestational diabetes to a dietitian. You will be taught to test your own blood glucose, and advised to exercise regularly, with NICE naming "walking for 30 minutes after a meal" as the example.

The RCOG adds that you will be advised to have your baby in a hospital with a consultant-led maternity unit and a neonatal unit, and that your team may include a diabetes doctor, an obstetrician, a specialist nurse and midwife, and a dietitian.

The targets you will be asked to hit

These are the same for gestational and pre-existing diabetes. NICE recommendation 1.3.5 asks for capillary plasma glucose below:

fasting: 5.3 mmol/litre
1 hour after meals: 7.8 mmol/litre, or
2 hours after meals: 6.4 mmol/litre

with the caveat "if these are achievable without causing problematic hypoglycaemia". If you are on insulin, recommendation 1.3.6 also asks you to stay above 4 mmol/litre. If you are managing with diet and exercise or tablets alone, NICE asks for fasting and 1-hour post-meal testing daily.

When medication is added, and why the timing is fixed

The sequence in NG3 is unusually specific, which makes it easy to check against your own care:

Fasting plasma glucose below 7 mmol/litre at diagnosis: "offer a trial of diet and exercise changes" (1.2.18).
If targets are not met with diet and exercise "within 1 to 2 weeks, offer metformin" (1.2.19). Not months. One to two weeks.
If metformin is contraindicated or unacceptable, offer insulin (1.2.20). If targets are still missed on metformin, add insulin (1.2.21).
Fasting plasma glucose 7.0 mmol/litre or above at diagnosis: offer "immediate treatment with insulin, with or without metformin" alongside diet and exercise changes (1.2.22).
Fasting plasma glucose between 6.0 and 6.9 mmol/litre plus complications such as macrosomia or hydramnios: consider immediate insulin, with or without metformin (1.2.23).

Needing tablets or insulin is not a failure of willpower. NICE tells clinicians to explain up front that "most women with gestational diabetes will need oral blood glucose-lowering agents or insulin".

How your baby is monitored

Recommendation 1.3.34: "Offer pregnant women with diabetes ultrasound monitoring of fetal growth and amniotic fluid volume every 4 weeks from 28 to 36 weeks." Note that amniotic fluid is measured alongside growth, because polyhydramnios is one of the things gestational diabetes causes.

Routine monitoring of fetal wellbeing before 38 weeks - Dopplers, heart rate recordings, biophysical profiles - "is not recommended", unless there is a risk of fetal growth restriction (1.3.35). More scans are not automatically better care.

Timing of birth

NICE recommendation 1.4.4 is the one to hold onto: "Advise women with gestational diabetes to give birth no later than 40 weeks plus 6 days. Offer elective birth by induced labour or (if indicated) by caesarean section to women who have not given birth by this time." Earlier birth is considered if there are maternal or fetal complications (1.4.5). During labour, capillary glucose is checked hourly and kept between 4 and 7 mmol/litre.

After the birth

Blood glucose-lowering therapy stops immediately after birth (1.6.3). Your baby's blood glucose is tested at 2 to 4 hours old (1.5.3). Before you go home, your own glucose is checked to exclude persisting hyperglycaemia.

Then the follow-up that gets forgotten. NICE 1.6.11 asks for a fasting plasma glucose test 6 to 13 weeks after the birth, and after 13 weeks either a fasting glucose or an HbA1c. A 75 g OGTT is specifically not offered routinely. The results are read as follows: fasting glucose below 6.0 mmol/litre means a low probability of diabetes now but a moderate risk of type 2 diabetes and an annual test; 6.0 to 6.9 mmol/litre means high risk; 7.0 mmol/litre or above means you are likely to have type 2 diabetes and need a confirmatory test. On HbA1c the bands are below 39 mmol/mol, 39 to 47 mmol/mol, and 48 mmol/mol or above.

Put the annual blood test in your calendar. It is the single most useful thing this diagnosis leaves you with.

Gestational Diabetes