Glycosylated haemoglobin (HbA1c) in pregnancy is a key measure of long-term blood glucose control, particularly relevant for women with pre-existing type 1 or type 2 diabetes. By reflecting average glucose levels over the preceding two to three months, HbA1c helps clinicians assess glycaemic management before conception and in early pregnancy, where poor control carries significant risks for both mother and baby. However, the physiological changes of pregnancy affect its reliability, meaning HbA1c must be interpreted carefully alongside other monitoring tools. This article explains what HbA1c measures, NICE targets, its limitations in pregnancy, and how blood glucose should be managed throughout.

What HbA1c Measures and Why It Matters in Pregnancy

HbA1c measures the proportion of haemoglobin bound to glucose, reflecting average blood glucose over two to three months; in pregnancy, it is primarily used to assess glycaemic control in women with pre-existing type 1 or type 2 diabetes, not to diagnose gestational diabetes.

Glycosylated haemoglobin, commonly referred to as HbA1c, is a blood test that reflects average blood glucose levels over the preceding two to three months. It works by measuring the proportion of haemoglobin — the oxygen-carrying protein in red blood cells — that has become chemically bound to glucose through a process called glycation. The higher the blood glucose over time, the greater the percentage of glycated haemoglobin detected.

In the context of pregnancy, HbA1c is primarily relevant for women with pre-existing type 1 or type 2 diabetes, both before conception and during the antenatal period. It is important to note that HbA1c is not used to diagnose or routinely monitor gestational diabetes mellitus (GDM); the oral glucose tolerance test (OGTT) is the standard diagnostic tool for GDM, and self-monitored blood glucose (SMBG) or continuous glucose monitoring (CGM) guide day-to-day management.

At the booking visit, HbA1c (or fasting plasma glucose) may help identify women with previously undiagnosed pre-existing diabetes, as recommended by NICE guideline NG3 (Diabetes in Pregnancy). Because poor glycaemic control during pregnancy is associated with a range of serious complications for both mother and baby, establishing a reliable baseline measure of glucose management is clinically essential.

HbA1c is expressed as a percentage or in millimoles per mole (mmol/mol), with the latter being the standard unit used across NHS laboratories in the UK. For example, an HbA1c of 48 mmol/mol (6.5%) or above is generally used to diagnose type 2 diabetes outside of pregnancy (NICE NG28). During pregnancy, however, the interpretation of HbA1c values requires additional nuance, as physiological changes can significantly affect the reliability of the result — a point explored in detail later in this article.

For women planning a pregnancy who already have diabetes, HbA1c measurement before conception is strongly recommended. Achieving optimal glycaemic control prior to conception reduces the risk of congenital anomalies and other adverse outcomes, making pre-pregnancy HbA1c one of the most clinically meaningful values in obstetric care.

NICE Guidelines on HbA1c Targets for Pregnant Women

NICE NG3 recommends women with pre-existing diabetes aim for HbA1c below 48 mmol/mol (6.5%) before conception and during pregnancy; an HbA1c above 86 mmol/mol (10%) is an indication to delay pregnancy until better control is achieved.

NICE guideline NG3 (Diabetes in Pregnancy) provides clear guidance on HbA1c targets for women with diabetes who are pregnant or planning to become pregnant. Women with pre-existing diabetes who are planning a pregnancy are advised to aim for an HbA1c level below 48 mmol/mol (6.5%) before conception, provided this can be achieved safely without problematic hypoglycaemia.

For women who are already pregnant and have pre-existing type 1 or type 2 diabetes, NICE recommends maintaining HbA1c below 48 mmol/mol (6.5%) throughout pregnancy, particularly during the first trimester when organogenesis is occurring. If HbA1c is above 86 mmol/mol (10%), NICE advises that women should be strongly advised not to become pregnant until better glycaemic control is achieved, given the substantially elevated risk of congenital malformations and miscarriage.

Key NICE NG3 recommendations include:

• Pre-conception: Aim for HbA1c below 48 mmol/mol; folic acid supplementation at 5 mg daily should begin at least three months before conception.

• During pregnancy: HbA1c should be measured early in pregnancy to inform risk assessment and thereafter as clinically indicated, taking into account its reduced reliability as gestation advances (see the following section). There is no fixed minimum measurement schedule specified by NICE beyond early pregnancy assessment.

• Gestational diabetes: HbA1c is not used to diagnose or routinely monitor GDM. The OGTT (using NICE-specified thresholds: fasting plasma glucose ≥5.6 mmol/L or 2-hour plasma glucose ≥7.8 mmol/L) is the preferred diagnostic tool.

NICE acknowledges the physiological limitations of HbA1c during pregnancy and therefore recommends that SMBG and, where appropriate, real-time CGM remain the primary tools for day-to-day glycaemic management. HbA1c in pregnancy should be interpreted alongside, rather than instead of, these more immediate glucose measurements. Further detail is available in NICE quality standard QS109 (Diabetes in Pregnancy).

Limitations of HbA1c Testing During Pregnancy

HbA1c reliability is reduced in pregnancy primarily due to increased red blood cell turnover, which can cause falsely lower readings; iron deficiency anaemia and haemoglobin variants can also affect accuracy depending on the assay method used.

Whilst HbA1c is a valuable marker of long-term glycaemic control, its reliability is significantly affected by the physiological changes that occur during pregnancy. Understanding these limitations is essential for both clinicians and patients to avoid misinterpretation of results.

One of the most important factors is the increased red blood cell turnover that occurs in pregnancy. As red blood cell production accelerates to meet increased physiological demands, red blood cells have a shorter lifespan than the usual approximately 120 days. Because HbA1c reflects glycation accumulated over the life of a red blood cell, a faster turnover means that HbA1c values may be artificially lower than the true average glucose level. This can give a falsely reassuring picture of glycaemic control, particularly in the second and third trimesters.

Additional factors that can affect HbA1c accuracy in pregnancy include:

• Iron deficiency anaemia, which is common in pregnancy and can falsely elevate HbA1c in some assay methods.

• Haemoglobin variants (such as HbS or HbC), which may interfere with certain laboratory measurement techniques. The degree of interference depends on the assay method used (for example, HPLC versus immunoassay); local laboratories will typically flag results where assay reliability may be affected and can advise on alternative methods.

Because of these confounding variables, NICE NG3 acknowledges that HbA1c has reduced diagnostic utility during pregnancy compared with outside of it. Clinicians are therefore advised not to rely solely on HbA1c for monitoring glycaemic control in pregnant women with diabetes. Instead, it should be used as one component of a broader assessment that includes fasting and postprandial glucose readings, real-time CGM where appropriate, and clinical review.

Risks Associated With Poorly Controlled Blood Sugar in Pregnancy

Poorly controlled blood glucose in pregnancy is associated with congenital malformations, macrosomia, stillbirth, neonatal hypoglycaemia, pre-eclampsia, and worsening of maternal diabetic complications such as retinopathy and nephropathy.

Suboptimal glycaemic control during pregnancy — whether reflected by elevated HbA1c or persistently high blood glucose readings — is associated with a range of serious complications affecting both the mother and the developing baby. The risks are broadly proportional to the degree and duration of hyperglycaemia.

Risks to the baby include:

• Congenital malformations: Elevated HbA1c in the first trimester, when fetal organ development is occurring, is associated with an increased risk of cardiac, neural tube, and other structural defects.

• Macrosomia: Excess glucose crosses the placenta, stimulating fetal insulin production and promoting excessive fetal growth, which can complicate delivery.

• Stillbirth: The risk of unexplained stillbirth is significantly higher in pregnancies complicated by poorly controlled diabetes.

• Neonatal hypoglycaemia: Babies born to mothers with poorly controlled diabetes may experience dangerously low blood sugar shortly after birth due to elevated fetal insulin levels.

• Preterm birth and respiratory distress syndrome are also more common in this group.

Risks to the mother include:

• Worsening of pre-existing diabetic complications, including retinopathy and nephropathy. NICE NG3 recommends that women with pre-existing diabetes have retinal assessment before conception and again early in pregnancy, with follow-up according to findings.

• Increased risk of pre-eclampsia and urinary tract infections.

• Higher rates of caesarean section due to fetal macrosomia or obstetric complications.

Diabetic ketoacidosis (DKA): Women with type 1 diabetes should be aware that DKA can develop rapidly in pregnancy and may occur at lower blood glucose levels than outside of pregnancy. Warning signs include persistent vomiting, abdominal pain, rapid or laboured breathing, and markedly elevated blood glucose with positive blood ketones. These symptoms require urgent same-day medical assessment or attendance at A&E. Women with type 1 diabetes should test blood ketones when unwell or when blood glucose is persistently above 11 mmol/L, in line with local sick-day rules agreed with their diabetes team.

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Women should contact their diabetes care team or midwife promptly if they experience persistent hyperglycaemia, symptoms of hypoglycaemia, reduced fetal movements, or any signs of infection. Early intervention is key to minimising risk, and multidisciplinary care involving an obstetrician, diabetologist, and specialist midwife is strongly recommended throughout pregnancy.

Monitoring and Managing Blood Glucose Alongside HbA1c

NICE NG3 recommends real-time CGM for pregnant women with type 1 diabetes and sets specific blood glucose targets, including a fasting level of 5.3 mmol/L and a one-hour post-meal level of 7.8 mmol/L, with insulin as the cornerstone of management.

Given the limitations of HbA1c during pregnancy, day-to-day blood glucose monitoring plays a central role in managing diabetes safely. NICE NG3 recommends that pregnant women with type 1 or type 2 diabetes monitor their blood glucose levels regularly, including fasting, pre-meal, one-hour post-meal, and bedtime readings where clinically indicated.

Target blood glucose levels recommended by NICE NG3 during pregnancy are:

• Fasting: 5.3 mmol/L

• One hour after meals: 7.8 mmol/L

• Two hours after meals: 6.4 mmol/L

For women with type 1 diabetes, NICE NG3 recommends offering real-time continuous glucose monitoring (rtCGM) as standard care throughout pregnancy. rtCGM provides real-time glucose data and trend information, enabling more responsive adjustments to insulin therapy and reducing the risk of both hyperglycaemia and hypoglycaemia. Evidence supports that rtCGM use in pregnant women with type 1 diabetes is associated with improved neonatal outcomes and reduced rates of large-for-gestational-age babies. Device choice and access should be guided by local pathways. Women with type 1 diabetes should also have blood ketone testing strips available and should test when unwell or when blood glucose is persistently elevated, following their agreed sick-day rules.

Management strategies may include:

• Insulin therapy: The cornerstone of glucose management in type 1 diabetes and commonly used in type 2 diabetes and GDM when blood glucose targets are not met with lifestyle measures or oral medication. Insulin does not cross the placenta and is considered safe in pregnancy.

• Metformin: May be used in some women with type 2 diabetes or GDM, in line with NICE NG3. Women should be informed that metformin does cross the placenta and that, whilst the available evidence is reassuring, long-term follow-up data in offspring are still being gathered. The decision to use metformin should be made jointly with the woman after discussion of benefits and risks. If you experience any suspected side effects from metformin or any other medicine, you can report these via the MHRA Yellow Card scheme (available at yellowcard.mhra.gov.uk).

• Dietary modification and physical activity: Fundamental components of GDM management, guided by a registered dietitian.

Women should attend all scheduled antenatal appointments and maintain open communication with their multidisciplinary diabetes team. Any unexplained deterioration in glucose control, recurrent hypoglycaemia, or concerns about fetal wellbeing should prompt timely clinical review.

After pregnancy: Women who had GDM should be offered a fasting plasma glucose test at the six-week postnatal check and annually thereafter, as recommended by NICE NG3, to detect type 2 diabetes or impaired glucose regulation. Women with pre-existing diabetes should continue structured follow-up with their diabetes team following delivery.

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