Does breastfeeding affect HbA1c? For women with diabetes or a history of gestational diabetes, this is a clinically important question. Lactation places significant metabolic demands on the body, diverting glucose towards milk production and potentially influencing insulin sensitivity — both of which can affect glycaemic markers over time. However, interpreting HbA1c during the postpartum period is not straightforward, as physiological changes in red blood cell turnover can alter the reliability of results. This article explores the evidence, explains how to interpret HbA1c during lactation, and outlines key management considerations for breastfeeding women with diabetes.

How Breastfeeding Influences Blood Glucose and Insulin Sensitivity

Breastfeeding increases glucose utilisation for milk production and may improve insulin sensitivity via prolactin, potentially lowering blood glucose — but also raising the risk of hypoglycaemia in insulin-treated women.

Breastfeeding has well-recognised metabolic effects that extend beyond simple calorie expenditure. During lactation, the body diverts glucose and fatty acids towards milk production — a process estimated to require an additional 400–500 kcal per day. This increased energy demand may contribute to lower circulating blood glucose levels in breastfeeding women compared with those who are not lactating, particularly in the hours following a feed.

From a hormonal perspective, prolactin — the primary hormone driving milk production — has been studied for potential effects on insulin secretion and peripheral insulin sensitivity. Some small studies suggest that prolactin may enhance cellular responsiveness to insulin, though this mechanism is not yet firmly established in humans and the evidence should be interpreted cautiously. Where such an effect does occur, it may translate into improved glycaemic control during the breastfeeding period, which is particularly relevant for women with type 1 diabetes, type 2 diabetes, or those who had gestational diabetes mellitus (GDM).

It is also worth noting that breastfeeding women may experience:

• Reduced fasting glucose levels due to continuous glucose utilisation for milk synthesis

• Greater variability in blood glucose, especially overnight, which can increase the risk of hypoglycaemia in insulin-treated women

• Gradual improvements in insulin sensitivity that may persist for several months postpartum, though the duration and magnitude vary between individuals

NICE guidance (NG3) highlights the increased risk of hypoglycaemia during and after breastfeeding in women using insulin. Women should be advised to have a fast-acting carbohydrate source readily available before and during feeds. These physiological changes are generally considered beneficial for long-term metabolic health, but they require careful monitoring — particularly for women managing diabetes with insulin or other glucose-lowering medications.

What the Evidence Says About HbA1c Levels During Lactation

Observational studies suggest breastfeeding may be associated with lower HbA1c in the postpartum period, but evidence is limited by small sample sizes and confounding; no regulatory body has quantified the expected change.

The question of whether breastfeeding directly affects HbA1c is nuanced, and the available evidence — whilst not yet definitive — points towards a possible modest association. HbA1c reflects average blood glucose over approximately 8–12 weeks by measuring the proportion of glycated haemoglobin in red blood cells. Any factor that alters glucose metabolism over this period, including lactation, has the potential to influence the result.

Some observational studies have reported lower HbA1c values in breastfeeding women with type 1 diabetes compared with non-breastfeeding counterparts in the early postpartum period. A frequently cited mechanism is the sustained glucose drain associated with milk synthesis, which may reduce average blood glucose concentrations over time. In women with a history of GDM, breastfeeding has been associated with a reduced risk of progression to type 2 diabetes, and some studies have noted more favourable HbA1c trajectories in those who breastfed for longer durations. However, these studies are largely observational, often involve small sample sizes, and are subject to confounding factors such as diet, physical activity, and pre-pregnancy BMI.

It is important to acknowledge the limitations of this evidence base:

• Many studies are observational and subject to confounding; causality cannot be assumed

• Sample sizes are often small, and follow-up periods vary considerably

• There is no official regulatory guidance from bodies such as NICE or the MHRA specifically quantifying the expected HbA1c change attributable to breastfeeding alone

A further important point: NICE (NG3) advises that HbA1c should not be used to diagnose diabetes during pregnancy or within the first 13 weeks postpartum, as physiological changes affecting red blood cell turnover can render the result unreliable for diagnostic purposes. Fasting plasma glucose is the preferred test for postpartum screening in this period.

The overall direction of evidence suggests that lactation may be associated with improved glycaemic markers in the postpartum period. Clinicians and patients should interpret this as a potentially beneficial but variable effect, rather than a guaranteed or predictable reduction in HbA1c.

Interpreting HbA1c Results in Breastfeeding Women

HbA1c can be artificially lowered by increased red blood cell turnover after delivery, or falsely elevated by iron deficiency anaemia; results should always be interpreted alongside SMBG or CGM data.

Interpreting HbA1c in breastfeeding women requires awareness of several physiological factors that can affect the accuracy and clinical meaning of the result.

One important consideration is that pregnancy and the early postpartum period are associated with increased red blood cell turnover. A higher proportion of younger red blood cells — which have had less time to become glycated — can artificially lower HbA1c, potentially underestimating true average glucose levels. Significant blood loss during delivery can have a similar effect. This phenomenon may persist for several weeks after delivery, and HbA1c results during this window should be interpreted alongside self-monitored blood glucose (SMBG) records or continuous glucose monitoring (CGM) data rather than in isolation.

Iron deficiency anaemia, which is relatively common in the postpartum period, can also influence HbA1c measurements. Iron deficiency typically causes a falsely elevated HbA1c, because iron-deficient red blood cells have a longer lifespan and therefore accumulate more glycation. Conversely, treatment of iron deficiency anaemia — by increasing red cell turnover — can lead to a fall in HbA1c that does not reflect a genuine improvement in glycaemic control. Women with suspected iron deficiency should have their iron status assessed alongside HbA1c to ensure accurate interpretation.

For women with diabetes, a lower-than-expected HbA1c during breastfeeding should not automatically be taken as a sign of optimal control. It may instead reflect:

• Increased hypoglycaemic episodes, particularly nocturnal, which lower the average without representing safe glucose management

• Physiological changes in red cell lifespan related to postpartum blood loss or increased turnover, rather than genuine improvements in glycaemia

• Dietary modifications made during the postpartum period

Clinicians should therefore use HbA1c alongside SMBG or CGM data to build a complete picture. NICE guidance (NG3, NG28) emphasises the importance of individualised targets and regular review, rather than relying on a single laboratory value in isolation. For postpartum screening after GDM, fasting plasma glucose — not HbA1c — is the recommended diagnostic test within the first 13 weeks.

Diabetes Management and Monitoring While Breastfeeding

Insulin requirements often fall during breastfeeding; insulin and metformin are safe during lactation, whilst sulfonylureas, SGLT2 inhibitors, and GLP-1 receptor agonists are not recommended per UK guidance.

For women with pre-existing type 1 or type 2 diabetes, breastfeeding introduces specific management considerations that differ from the non-lactating postpartum period. Insulin requirements often fall significantly after delivery and may decrease further during active breastfeeding due to enhanced insulin sensitivity. Women using insulin should be counselled about the increased risk of hypoglycaemia, particularly during and immediately after feeds, and should be advised to have a fast-acting carbohydrate source readily available at all times (NICE NG3).

NICE guidance (NG3) recommends that women with diabetes who are breastfeeding receive ongoing support from a specialist diabetes team, including regular review of insulin doses and glucose targets. Blood glucose targets in the postpartum period are generally slightly less strict than during pregnancy, but women should still aim for good overall control to support their own health and energy levels during a demanding time.

With regard to medications, safety during breastfeeding is an important consideration. Based on NICE NG3, the BNF, and the UK Drugs in Lactation Advisory Service (UKDILAS/SPS):

• Insulin (all types) is considered safe during breastfeeding and does not pass into breast milk in clinically significant amounts; it remains the preferred treatment for type 1 diabetes and insulin-requiring type 2 diabetes during lactation

• Metformin is generally considered compatible with breastfeeding and is supported by NICE NG3 and the BNF; individual clinical judgement applies

• Sulfonylureas and DPP-4 inhibitors are generally not recommended during breastfeeding per UK guidance (BNF, SPS UKDILAS)

• SGLT2 inhibitors and GLP-1 receptor agonists are not recommended during breastfeeding due to insufficient safety data; product information (SmPCs) for these agents typically advises against use during lactation

Women who had GDM should be offered a fasting plasma glucose test at 6–13 weeks postpartum, as recommended by NICE NG3, to screen for persistent glucose abnormalities. Routine use of an oral glucose tolerance test (OGTT) is not recommended by NICE in this context. Breastfeeding does not preclude this testing, though timing relative to feeds may need to be considered. Women with a history of GDM should also be offered annual diabetes screening thereafter, as advised by NICE.

If you experience any suspected side effects from diabetes medications whilst breastfeeding, you can report these via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

When to Seek Advice From Your GP or Diabetes Team

Women should seek urgent medical advice for frequent hypoglycaemia, persistent hyperglycaemia, or any concerns about medication safety during breastfeeding, and should not adjust insulin doses independently.

Whilst breastfeeding can have beneficial effects on blood glucose and potentially on HbA1c, it also introduces new challenges that warrant prompt professional input in certain circumstances. Women should not attempt to adjust insulin doses or other diabetes medications independently based on perceived changes in glucose levels during lactation. Any significant or sustained change in blood glucose patterns should be discussed with a GP or specialist diabetes team.

Seek emergency help immediately (call 999) if you experience:

• Severe hypoglycaemia requiring assistance from another person, loss of consciousness, or a seizure

• Symptoms of diabetic ketoacidosis (DKA), which may include abdominal pain, vomiting, rapid or laboured breathing, drowsiness, or a fruity smell on the breath — this is a medical emergency

Contact NHS 111 or your GP urgently if you experience:

• Frequent or severe hypoglycaemia, particularly if occurring overnight or without adequate warning symptoms

• Unexpectedly high blood glucose readings that persist despite usual management, which may indicate a need for medication review

• Symptoms of hyperglycaemia such as excessive thirst, frequent urination, or fatigue that is disproportionate to the demands of new parenthood

• Concerns about the safety of any medication whilst breastfeeding, including questions about newly prescribed treatments

Women who had gestational diabetes and have not yet had their postpartum glucose screening should contact their GP to arrange a fasting plasma glucose test, ideally within 13 weeks of delivery, as recommended by NICE NG3. Early identification of impaired fasting glucose or type 2 diabetes allows for timely lifestyle intervention, which is particularly effective in this population.

Breastfeeding support and diabetes care should not be managed in isolation. Midwives, health visitors, diabetes specialist nurses, and GPs all play a role in supporting women through this period. If at any point a woman feels uncertain about her glucose management, her HbA1c result, or the safety of her medications, she should feel empowered to seek a review without delay. Good communication between the woman and her healthcare team remains the cornerstone of safe and effective diabetes management during lactation.

Frequently Asked Questions