Does HbA1c increase during pregnancy? Contrary to what many might expect, HbA1c levels typically fall during pregnancy rather than rise. This is due to increased red blood cell turnover, which shortens the lifespan of red blood cells and reduces the time available for glucose to attach to haemoglobin. Understanding how pregnancy affects HbA1c is essential for women with pre-existing diabetes or gestational diabetes, as misinterpreting results can lead to inadequate monitoring. This article explains the physiology behind HbA1c changes in pregnancy, what NICE guidelines recommend, and which blood glucose tests are most reliable.

How Pregnancy Affects HbA1c Levels

HbA1c typically falls during pregnancy due to increased red blood cell turnover, which shortens RBC lifespan and reduces glucose accumulation on haemoglobin, making it a less reliable standalone glycaemic marker.

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During pregnancy, the body undergoes significant physiological changes that directly influence HbA1c readings. HbA1c (glycated haemoglobin) measures the percentage of haemoglobin that has glucose attached to it, reflecting average blood glucose levels over the preceding two to three months. However, pregnancy alters this measurement in ways that are important to understand.

The principal reason HbA1c tends to fall during pregnancy is an increase in red blood cell (RBC) turnover. Pregnancy stimulates increased erythropoiesis — the production of new red blood cells — to support the growing foetus and meet greater circulatory demands. Because HbA1c accumulates on red blood cells over their lifespan, a shorter RBC lifespan means there is less time for glucose to attach. As a result, HbA1c levels typically fall during pregnancy, even if blood glucose control has not meaningfully improved. This is the dominant physiological mechanism and is well recognised in NICE guidance (NG3: Diabetes in Pregnancy).

In women with iron deficiency anaemia — which is common in pregnancy — erythropoiesis may be impaired, leading to an older RBC population with a prolonged lifespan. This can cause HbA1c to appear falsely elevated relative to true glycaemic control. Clinicians should be aware of this when interpreting results in anaemic patients.

In summary, rather than consistently increasing, HbA1c tends to decrease during a healthy pregnancy due to increased red blood cell turnover and shortened RBC lifespan. This makes it a less reliable standalone marker for monitoring blood glucose in pregnant women, and results must always be interpreted carefully within the broader clinical context (NICE NG3).

Why HbA1c Can Be Misleading During Pregnancy

HbA1c cannot capture postprandial glucose spikes or rapid fluctuations, and physiological RBC changes may produce falsely reassuring results, making it insufficient as a primary monitoring tool in pregnancy.

The limitations of HbA1c during pregnancy are well recognised by diabetes specialists and obstetric teams. Because the test reflects an average blood glucose level over two to three months, it cannot capture the rapid fluctuations in blood sugar that are particularly significant during pregnancy — especially the post-meal (postprandial) rises that are associated with adverse outcomes including macrosomia (large baby) and other foetal complications (NICE NG3).

Pregnancy is characterised by insulin resistance, particularly in the second and third trimesters, driven by placental hormones such as human placental lactogen, oestrogen, and cortisol. This resistance means blood glucose can rise sharply after meals even when fasting levels appear normal. HbA1c would not adequately capture these peaks, potentially giving a falsely reassuring picture of glycaemic control.

Furthermore, the physiological reduction in HbA1c caused by increased red blood cell turnover means that even women with suboptimal glucose control may present with HbA1c values within or near the normal range. This can lead to under-recognition of gestational diabetes or inadequate monitoring of pre-existing diabetes during pregnancy.

It is also worth noting that certain haemoglobin variants — more prevalent in some ethnic groups — can interfere with HbA1c assays, producing inaccurate results depending on the analytical method used. Women and clinicians should be aware that the testing laboratory can advise on whether a specific assay is affected by haemoglobin variants. Given that the UK has a diverse population, this is a clinically relevant consideration.

For all these reasons, HbA1c alone is not considered a reliable or sufficient tool for diagnosing gestational diabetes or as the primary monitoring tool during pregnancy. NICE NG3 does, however, recommend its use at specific points in the care of women with pre-existing diabetes (see below), always alongside self-monitored blood glucose data. Alternative testing methods — particularly the oral glucose tolerance test and continuous glucose monitoring — are central to UK clinical practice.

What NICE Guidelines Say About HbA1c in Pregnancy

NICE NG3 recommends HbA1c at the booking appointment and around 28 weeks for pre-existing diabetes, but explicitly states it must not be used to diagnose gestational diabetes mellitus.

NICE guideline NG3 (Diabetes in Pregnancy: Management from Preconception to the Postnatal Period) provides clear direction on the use of HbA1c during pregnancy. Whilst HbA1c is a cornerstone of diabetes management outside of pregnancy, its role during pregnancy is more limited and context-specific.

Preconception guidance: For women with pre-existing type 1 or type 2 diabetes who are planning a pregnancy, NICE advises that an HbA1c above 86 mmol/mol (10%) indicates a level of risk at which conception should be deferred until glycaemic control improves. This is a preconception recommendation and does not apply once pregnancy is established.

During pregnancy — pre-existing diabetes: For women with pre-existing type 1 or type 2 diabetes, NICE recommends that HbA1c is measured at the booking appointment (ideally before 10 weeks of gestation) to provide a baseline assessment of glycaemic control. An HbA1c above 48 mmol/mol (6.5%) at booking is associated with an increased risk of congenital malformations, miscarriage, and other adverse outcomes. NICE also advises that HbA1c may be checked at around 28 weeks to inform ongoing management, but it should always be interpreted alongside self-monitored blood glucose (SMBG) or continuous glucose monitoring (CGM) data, given the physiological factors that reduce its accuracy in pregnancy.

NICE does not set a specific HbA1c target of 42 mmol/mol (6%) during pregnancy. The emphasis in pregnancy is on achieving SMBG and CGM targets (see below), with HbA1c kept as low as safely achievable — generally below 48 mmol/mol — whilst acknowledging that physiological changes will naturally lower the reading.

Gestational diabetes: NICE explicitly states that HbA1c should not be used to diagnose gestational diabetes mellitus (GDM) during pregnancy, due to the physiological factors that reduce its accuracy. The oral glucose tolerance test (OGTT) is the recommended diagnostic method (NICE NG3).

Which Blood Sugar Tests Are Recommended During Pregnancy

The OGTT is the gold standard for diagnosing gestational diabetes in the UK, with self-monitored blood glucose and real-time CGM (for type 1 diabetes) used for ongoing management.

Given the limitations of HbA1c, a combination of blood glucose monitoring methods is used during pregnancy to ensure accurate and timely assessment of glycaemic control.

Oral Glucose Tolerance Test (OGTT): The OGTT remains the gold standard for diagnosing gestational diabetes in the UK. It involves fasting overnight, having a blood sample taken, drinking a 75 g glucose solution, and having a further blood sample taken two hours later. According to NICE NG3, GDM is diagnosed if:

• Fasting plasma glucose is 5.6 mmol/L or above, or

• The 2-hour value is 7.8 mmol/L or above

The OGTT is typically offered between 24 and 28 weeks of pregnancy to women with risk factors. Earlier testing may be offered to women at higher risk — for example, those with a previous diagnosis of GDM or marked risk factors — in line with local pathways under NICE NG3.

Self-Monitoring of Blood Glucose (SMBG): Women with pre-existing diabetes or confirmed GDM are advised to monitor their blood glucose regularly at home using a glucometer. NICE NG3 recommends testing:

• Fasting (on waking)

• One hour after meals

Target levels are: fasting below 5.3 mmol/L, and one hour post-meal below 7.8 mmol/L. For women who monitor at two hours after meals, the target is below 6.4 mmol/L (NICE NG3).

Continuous Glucose Monitoring (CGM): NICE NG3 recommends that real-time CGM is offered to all pregnant women with type 1 diabetes throughout pregnancy, as it provides detailed insight into glucose trends and reduces the risk of hypoglycaemia and adverse pregnancy outcomes. For women with type 2 diabetes or GDM, routine use of CGM is not currently recommended by NICE; use in these groups should be individualised and discussed with the diabetes team.

Together, these tools provide a far more comprehensive and clinically meaningful picture of blood glucose control than HbA1c alone during pregnancy.

Managing Blood Glucose Safely When Pregnant

Gestational diabetes is managed first with dietary changes and physical activity; metformin or insulin is initiated if targets are not met, with care delivered by a multidisciplinary team.

Maintaining safe blood glucose levels during pregnancy is essential for both maternal and foetal wellbeing. Poorly controlled blood sugar — whether from pre-existing diabetes or gestational diabetes — is associated with risks including macrosomia (large baby), pre-eclampsia, preterm birth, neonatal hypoglycaemia, and stillbirth. Conversely, overly aggressive glucose lowering can cause hypoglycaemia, which carries its own risks.

Dietary and lifestyle measures form the first line of management for gestational diabetes. Women are typically advised to:

• Follow a low glycaemic index (GI) diet, rich in vegetables, wholegrains, and lean protein

• Avoid sugary drinks and refined carbohydrates

• Engage in regular moderate physical activity, such as walking after meals, which can significantly improve insulin sensitivity

• Distribute carbohydrate intake evenly across meals and snacks throughout the day

Pharmacological treatment may be initiated if blood glucose targets are not met within one to two weeks of dietary changes, or sooner if clinically indicated. NICE NG3 provides the following guidance:

• Metformin is recommended as a first-line oral agent for GDM where not contraindicated.

• Insulin should be considered immediately (without a trial of metformin) if fasting plasma glucose at diagnosis is 7.0 mmol/L or above. Insulin should also be considered if fasting plasma glucose is 6.0–6.9 mmol/L and there is evidence of complications such as macrosomia or polyhydramnios.

• If metformin is not tolerated and the woman declines insulin, glibenclamide may be offered as an alternative oral agent (NICE NG3).

• Women with pre-existing type 2 diabetes may continue metformin during pregnancy as an adjunct or alternative to insulin; other oral hypoglycaemic agents are generally discontinued (NICE NG3; BNF).

• Women with pre-existing type 1 diabetes will continue insulin therapy, with doses adjusted throughout pregnancy under specialist supervision.

Women with diabetes in pregnancy are also advised to take folic acid 5 mg daily from pre-conception until 12 weeks to reduce the risk of neural tube defects, and vitamin D supplementation throughout pregnancy in line with NHS guidance. Women with diabetes are at increased risk of pre-eclampsia; NICE and RCOG recommend low-dose aspirin (75–150 mg at night) from 12 weeks of pregnancy for those at elevated risk — women should discuss this with their obstetric team.

Care should be delivered by a multidisciplinary diabetes in pregnancy team, typically including a diabetes specialist nurse, dietitian, obstetrician, and midwife.

When to Speak to Your Midwife or Diabetes Team

Women should contact their midwife or diabetes team promptly if blood glucose readings are consistently outside target range, or if they experience hypoglycaemia, hyperglycaemia symptoms, or concerns about foetal movement.

Knowing when to seek advice is a vital part of safe diabetes management during pregnancy. Women should feel empowered to contact their midwife, GP, or diabetes team promptly if they have any concerns — early intervention can prevent complications from escalating.

Contact your midwife or diabetes team if you experience:

• Blood glucose readings that are consistently above or below your agreed target range

• Frequent or severe hypoglycaemic episodes (symptoms include shakiness, sweating, confusion, or loss of consciousness)

• Symptoms of hyperglycaemia, such as excessive thirst, frequent urination, fatigue, or blurred vision

• Difficulty managing your diet or understanding your glucose monitoring results

• Any concerns about your baby's movements, as reduced foetal movement can sometimes be associated with poorly controlled diabetes

Call 999 or go to your nearest A&E immediately if you experience symptoms of diabetic ketoacidosis (DKA) or severe hypoglycaemia. DKA — although more common in type 1 diabetes — can develop rapidly during pregnancy and is a medical emergency. Symptoms include nausea, vomiting, abdominal pain, rapid breathing, and a fruity smell on the breath. Do not wait to contact your diabetes team if you suspect DKA; seek emergency care without delay (NHS).

Postnatal follow-up: Women diagnosed with gestational diabetes should be aware that their condition requires postnatal follow-up. NICE NG3 recommends a fasting plasma glucose test at the 6–13 week postnatal check to screen for type 2 diabetes. If this is not possible within that window, an HbA1c test at 13 weeks or later is an acceptable alternative. Annual HbA1c testing is recommended thereafter, as GDM significantly increases the lifetime risk of developing type 2 diabetes.

Open communication with your healthcare team throughout pregnancy is essential. Do not hesitate to raise questions about your HbA1c results, glucose targets, or medication — your team is there to support you in achieving the safest possible outcome for you and your baby.

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