Gestational diabetes OGTT threshold checker

Two women, the same blood, different answers

Take a fasting glucose of 5.3 millimoles per litre at 26 weeks. In Britain that is a normal result and the woman goes home. In much of Europe, in Australia, and under the American Diabetes Association's one-step approach, the same number is a diagnosis of gestational diabetes, with finger-pricks, dietitian appointments, extra scans and a different birth plan attached.

Nothing about the blood changed. The line moved.

This is the single most confusing thing about gestational diabetes, and almost nobody explains it, which is why women compare results in forums and conclude that someone must be wrong. Nobody is. There are three sets of thresholds in current use and they genuinely disagree.

Where the disagreement comes from

The WHO and IADPSG numbers come from the HAPO study, which measured glucose in tens of thousands of pregnancies and looked at outcomes. The trouble it found is that the relationship between blood glucose and adverse outcome is a smooth curve. There is no point on it where risk suddenly jumps. So a threshold has to be chosen rather than discovered, and IADPSG chose the level carrying 1.75 times the odds of an adverse outcome.

NICE looked at the same underlying problem and ran its own health-economic modelling: how many women would be diagnosed at each threshold, what treating them would cost, and how much benefit it would buy. It landed on 5.6 fasting and 7.8 at two hours. ACOG kept a two-step process that has been in American practice for decades and requires two abnormal values rather than one.

Three defensible answers to the question "where should the line go". Not three measurement errors.

The two gaps this creates

The first is the one people notice: a fasting result between 5.1 and 5.5. Diagnostic under WHO, normal under NICE.

The second runs the other way and is much less discussed: a 2-hour result between 7.8 and 8.4. Diagnostic under NICE, normal under WHO. NICE is more lenient when fasting and stricter at two hours. So "the UK diagnoses fewer women" is too simple — it diagnoses a different set of women.

Why the test is so unpleasant

You fast overnight, arrive early, have blood taken, drink a very sweet drink, and then sit still for two hours with nothing to eat. Sitting still matters: walking about lowers your glucose and can change the result. Feeling nauseated afterwards is extremely common and is not a sign that anything is wrong. If you vomit the drink, tell the staff — the test will need repeating rather than being read as normal.

In the UK the test is offered at 24 to 28 weeks to women with risk factors. Anyone who had gestational diabetes in a previous pregnancy is offered either early home monitoring or a test soon after booking, with a repeat at 24 to 28 weeks if the first is normal.

The American route is a different test entirely

If you are reading American advice with a British result, or the reverse, the mismatch is bigger than the numbers. The United States mostly runs a two-step process. First a 50 gram glucose challenge that you do not fast for, which can be slotted into an ordinary appointment. Most women pass it and stop there. Roughly one in five are asked back for a 100 gram, three-hour fasting test, and that second test needs two abnormal values out of four before it diagnoses.

That two-of-four rule is doing a lot of work. Under the one-step criteria, a single abnormal value is a diagnosis. So an American woman with one borderline value is told she is fine and a European woman with the same single value is not. Studies have not shown a clear difference in outcomes between the approaches, which is why several methods remain acceptable rather than one winning.

The number that is not the diagnosis number

Here is where a home glucose meter goes wrong. Once you are diagnosed, the fasting target is below 5.3 — a lower number than the 5.6 that diagnosed you. Women see 5.4 on a meter, remember that the diagnostic threshold was 5.6, and conclude they are fine. They are above target.

The daily targets are: below 5.3 fasting, below 7.8 one hour after a meal, below 6.4 two hours after. If you are on insulin there is a floor as well, at 4.0. These are national targets and your own team may agree different individual ones with you; where they differ, theirs win.

What happens if you are diagnosed

You should be reviewed at a joint diabetes and antenatal clinic within a week. Most women start with changes to diet and exercise. If glucose targets are not met within one to two weeks, metformin is offered, and insulin after or alongside it. If the fasting glucose at diagnosis was already 7.0 or above, insulin is offered immediately.

Needing medication is not a failure. Gestational diabetes is caused by placental hormones making you resistant to your own insulin, and the placenta does not care what you ate. Most women with it end up on something, and the ones who do not are not doing better; they had a milder version of the same physiology.

Afterwards

Gestational diabetes usually resolves after the birth, but it is a signpost. A substantial proportion of women who have had it go on to develop type 2 diabetes later, and guidelines recommend postpartum screening, usually 4 to 12 weeks after the birth, and regular checks after that. That appointment is the most valuable thing this diagnosis gives you, and it is the one most often skipped.

This tool compares numbers to published tables. It does not diagnose anything and does not replace assessment by a clinician.