An HbA1c Taken During Pregnancy
This test has two opposite roles at opposite ends of pregnancy, and confusing them causes real problems. Early on it is useful, because it can reveal diabetes that was already present and unrecognized. Later it is not used to diagnose gestational diabetes at all — pregnancy speeds up red cell turnover, which drags the number down, so a normal HbA1c in the second half of pregnancy is not evidence that glucose handling is fine.
The pattern on your report
- HbA1c Normal Key
- Fasting glucose High-normal Key
- Hemoglobin Low-normal Key
- Ferritin Low Key
Printed as: Fasting glucose in mmol/Lor mg/dL— Two scales differing roughly eighteenfold; check which your report uses.Ferritin in ug/Lor ng/mL— Iron deficiency lengthens red cell survival and pushes HbA1c up, partly masking the pregnancy effect.Hemoglobin in g/Lor g/dL— Falls with the dilution of pregnancy, which is part of what shifts the HbA1c down.HbA1c in mmol/molor %— Reported as mmol/mol by some laboratories and as a percentage by others, and the two figures look nothing alike for the same result.
Why the numbers look like this
HbA1c works by counting sugar attached to hemoglobin inside red cells. A cell that lives four months accumulates that attachment for four months, so the number reports an average weighted by how long the cells have been circulating.
Pregnancy changes both halves of that. Red cell production increases and cells are replaced faster, so the average cell in circulation is younger and has had less time to accumulate sugar. Plasma volume expands at the same time, diluting things further.
The result is an HbA1c that runs lower in pregnancy than the same glucose control would produce outside it. It falls furthest in the second trimester, which is precisely when gestational diabetes is screened for.
Iron deficiency pushes in the opposite direction, because iron-deficient red cells live longer and accumulate more sugar. Since iron deficiency is common in pregnancy, the two errors can partly cancel or can compound, and neither is visible on the report.
That is why the oral glucose tolerance test remains the diagnostic test in pregnancy. It measures the thing itself rather than a proxy whose relationship to it has shifted.
Not being flagged is not the same as normal
The thresholds printed beside an HbA1c are derived from non-pregnant adults and describe risk in that group. Applying them in the second half of pregnancy misclassifies people in the reassuring direction, which is the more dangerous error. Reported units differ too, with some laboratories giving mmol/mol and others a percentage, and the two figures look nothing alike for the same result. Early in pregnancy the test is being used for a different question entirely, so the same number carries a different meaning depending on when it was taken.
What else on the report can hide this
The oral glucose tolerance test is the test that answers the gestational diabetes question, and no HbA1c substitutes for it. If risk factors are present or a previous pregnancy was affected, it is arranged regardless of what an HbA1c showed.
An HbA1c in the first weeks is worth having for the opposite reason: a clearly raised value that early points to diabetes predating the pregnancy, which changes care substantially and needs specialist involvement from the start.
Ferritin belongs alongside any HbA1c in pregnancy, since iron deficiency shifts it upward and is common enough to matter.
Home glucose monitoring, where it has been done, carries more information than a single HbA1c and is not affected by red cell turnover at all.
After delivery the question changes again. Gestational diabetes carries a substantial ongoing risk of type 2 diabetes, so a check some weeks after the birth and then regularly afterwards is the part that most often gets dropped once the pregnancy is over.
What usually causes it
Listed from most to least common — not from most to least serious.
- Very common
Faster red cell turnover in pregnancy
Lowers the value against the same glucose control, most markedly in the second trimester. This is why the test is not used to diagnose gestational diabetes.
- Very common
Gestational diabetes with a normal HbA1c
Entirely possible and the reason the glucose tolerance test exists. A reassuring HbA1c does not exclude it.
- Very common
Iron deficiency
Pushes the value up, so it can partly mask the pregnancy effect. Common enough in pregnancy that ferritin belongs alongside.
- Common
Pre-existing type 2 diabetes
A clearly raised HbA1c in the first weeks. Changes antenatal care substantially and needs specialist involvement early.
- Common
Hemodilution
Plasma volume expands faster than red cell mass, contributing to the downward shift alongside the turnover effect.
- Uncommon
A hemoglobin variant
Interferes with some assay methods and shortens red cell survival, so the value can be unusable while looking ordinary.
- Uncommon
Recent blood loss or transfusion
Both change the age distribution of circulating cells and therefore the value, independently of glucose.
- Rare
Type 1 diabetes presenting in pregnancy
Thirst, weight loss and ketones, with glucose clearly high. This is an urgent presentation rather than a screening question.
What is usually checked next
- An oral glucose tolerance test The diagnostic test for gestational diabetes, and no HbA1c substitutes for it.
- An HbA1c in the first weeks of pregnancy Used for the other question: whether diabetes predated the pregnancy.
- Ferritin Iron deficiency shifts HbA1c upward, and it is common enough in pregnancy to change the interpretation.
- Home glucose monitoring where it is being done Measures the thing itself and is unaffected by red cell turnover.
- A check some weeks after delivery, and regularly afterwards Gestational diabetes carries a substantial later risk of type 2, and this step is the one most often dropped.
When to seek care sooner
- Emergency Vomiting with abdominal pain and rapid breathing
- Emergency Reduced fetal movements
- Emergency Severe headache, visual disturbance, or pain below the ribs on the right
- Same day Excessive thirst with passing large volumes of urine
- Same day Unintentional weight loss during pregnancy
- Soon A clearly raised HbA1c in the first weeks
Questions worth bringing to your appointment
- Should I have a glucose tolerance test regardless of this HbA1c?
- How many weeks pregnant was I when this was taken?
- Has my ferritin been checked alongside?
- Was an HbA1c done early to check for pre-existing diabetes?
- What follow-up should I have after the baby is born?