High Glucose During Pregnancy: Causes and Risks

High glucose during pregnancy, most commonly known as gestational diabetes mellitus (GDM), affects around 1 in 20 pregnancies in the UK and can carry significant risks for both mother and baby if left unmanaged. Because the condition rarely causes noticeable symptoms, NHS screening is offered to women identified as being at increased risk. Understanding what raised blood glucose means in pregnancy, how it is diagnosed, what complications can arise, and what happens after delivery can help women engage confidently with their antenatal care and make informed decisions throughout their pregnancy.

What does high blood glucose during pregnancy actually mean?

High blood glucose in pregnancy most commonly means gestational diabetes mellitus (GDM), caused by placental hormones reducing insulin sensitivity. It typically develops in the second or third trimester and affects around 1 in 20 UK pregnancies.

High blood glucose during pregnancy most commonly refers to gestational diabetes mellitus (GDM), a condition in which blood sugar levels rise above normal thresholds specifically because of the hormonal and metabolic changes that occur during pregnancy. It is distinct from pre-existing type 1 or type 2 diabetes, although women with those conditions may also experience worsening glucose control during pregnancy and require separate management pathways.

During pregnancy, the placenta produces hormones such as human placental lactogen that progressively reduce the body's sensitivity to insulin. In most women, the pancreas compensates by producing more insulin. In women who develop GDM, this compensation is insufficient, and blood glucose rises to levels that can affect both mother and baby. The condition typically develops in the second or third trimester, when placental hormone levels are highest.

GDM is more common than many people realise. NHS England maternity data indicate it affects around 1 in 20 pregnancies in the UK, though prevalence varies by population and local risk-factor distribution. Certain groups face a higher risk, as set out in NICE guideline NG3 (Diabetes in pregnancy), including women with a body mass index above 30 kg/m², those with a first-degree relative with type 2 diabetes, women from South Asian, Black African, African-Caribbean or Middle Eastern backgrounds, those who have had GDM in a previous pregnancy, and those who have previously given birth to a baby weighing 4.5 kg or more.

It is important to understand that GDM does not usually cause noticeable symptoms, which is why risk-based screening is offered on the NHS in line with NICE guidance. Most women feel entirely well, making antenatal testing the primary way the condition is identified.

How is gestational diabetes diagnosed on the NHS?

GDM is diagnosed via an OGTT, with a fasting plasma glucose of 5.6 mmol/L or above, or a two-hour level of 7.8 mmol/L or above, confirming the diagnosis. Testing is offered at 24 to 28 weeks for at-risk women.

Gestational diabetes is diagnosed on the NHS using an oral glucose tolerance test (OGTT), which NICE guideline NG3 recommends for women identified as being at increased risk. The OGTT is typically offered between 24 and 28 weeks of pregnancy. For women who have had GDM in a previous pregnancy, NICE NG3 advises that testing should be offered as soon as possible after the booking appointment; in practice many NHS trusts arrange this at around 16 to 18 weeks. If the result at that early test is normal, a repeat OGTT is still recommended at 24 to 28 weeks, as glucose tolerance can deteriorate as the pregnancy progresses.

The test involves fasting overnight for at least eight hours, having a blood sample taken to measure fasting plasma glucose, drinking a 75 g glucose solution, and then having a further blood sample taken two hours later.^[1][2] According to NICE NG3, GDM is diagnosed if the fasting plasma glucose is 5.6 mmol/L or above, or if the two-hour plasma glucose is 7.8 mmol/L or above. Either result alone is sufficient for a diagnosis.

Women who are not in a higher-risk group are not routinely offered the OGTT, as NICE recommends risk-based rather than universal screening. However, if a midwife or obstetrician identifies risk factors at the booking appointment or at any point during pregnancy, a referral for testing should be made promptly. Women who notice symptoms such as increased thirst, frequent urination or unusual fatigue should mention these to their midwife, although these symptoms are non-specific and common in pregnancy generally.

Once a diagnosis is confirmed, women are referred to a joint obstetric and diabetes clinic for specialist care. Management typically begins with dietary advice, blood glucose self-monitoring and physical activity guidance. NICE NG3 recommends that women aim for fasting capillary plasma glucose below 5.3 mmol/L and a one-hour post-meal level below 7.8 mmol/L (or two-hour level below 6.4 mmol/L). If these targets are not met after a short trial of lifestyle measures, medication is considered; NICE NG3 advises that this decision is usually made within one to two weeks of diagnosis if diet and activity changes alone are insufficient.^[1][3]

What are the risks of untreated high glucose for me and my baby?

Untreated GDM raises the risk of macrosomia, neonatal hypoglycaemia, shoulder dystocia, pre-eclampsia and stillbirth. Appropriate monitoring and treatment can substantially reduce these risks.

Untreated high blood glucose during pregnancy carries significant risks for both the mother and the baby, which is why early diagnosis and management are so important. For the baby, the most common complication is macrosomia, meaning the baby grows larger than expected because excess glucose crosses the placenta and stimulates the baby's own insulin production, promoting fat storage and accelerating growth. A larger baby increases the likelihood of a difficult labour, instrumental delivery or caesarean section.

Other risks to the baby include:

• Neonatal hypoglycaemia: After birth, the baby's insulin levels remain high while the glucose supply from the placenta stops abruptly, which can cause dangerously low blood sugar in the newborn.

• Polyhydramnios: An excess of amniotic fluid, which can cause discomfort and increase the risk of preterm labour.

• Neonatal jaundice: Newborns of mothers with GDM have a higher likelihood of developing jaundice in the days after birth, which may require monitoring or treatment.

• Preterm birth: GDM is associated with an increased risk of delivery before 37 weeks.

• Stillbirth: Although the absolute risk remains low, poorly controlled GDM is associated with an increased risk of late pregnancy loss.

• Shoulder dystocia: A larger baby may have difficulty passing through the birth canal, which is a medical emergency requiring skilled obstetric management.

For the mother, risks include pre-eclampsia (high blood pressure with organ involvement) and a higher rate of caesarean delivery. Women with GDM also have a substantially elevated lifetime risk of developing type 2 diabetes; NICE NG3 notes this risk is significantly increased compared with the general population, making long-term follow-up essential.^[1][14]

With appropriate monitoring and treatment, many of these risks can be substantially reduced. Women should attend all antenatal appointments and contact their midwife or maternity unit promptly if they have any concerns about their glucose readings. If you notice a reduction in your baby's movements, contact your maternity unit immediately for assessment; RCOG patient information on reduced fetal movements provides further guidance on when and how to seek urgent advice.

What happens to my blood glucose levels after I give birth?

Blood glucose usually returns to normal within days of delivery, but NICE NG3 recommends a fasting plasma glucose test at six to thirteen weeks postnatally and annual HbA1c checks thereafter due to elevated type 2 diabetes risk.

For most women with gestational diabetes, blood glucose levels return to normal within a few days of giving birth, once the placental hormones that caused insulin resistance are no longer present. However, this does not mean the condition can be forgotten entirely, as GDM is a strong indicator of underlying metabolic vulnerability.

NICE guideline NG3 recommends that women who have had GDM are offered a fasting plasma glucose test six to thirteen weeks after delivery to check whether glucose levels have normalised.^[1][4] This test is usually arranged by the GP, and it is important that information about the GDM diagnosis is shared between the maternity team and primary care so that follow-up is not missed. If the six-to-thirteen-week result is normal, NICE NG3 recommends annual HbA1c testing thereafter, as the long-term risk of developing type 2 diabetes remains significantly elevated. Women should be informed of this risk at diagnosis and supported to make lifestyle changes, including maintaining a healthy weight, eating a balanced diet and taking regular physical activity, all of which are evidence-based strategies for reducing progression to type 2 diabetes.

Breastfeeding is encouraged for women who have had GDM, as evidence suggests it is associated with improved glucose metabolism in the mother.^[7][8] Both metformin and insulin are considered compatible with breastfeeding; women who were taking either medicine during pregnancy should discuss with their clinical team whether these need to be continued after delivery, as requirements usually change rapidly once the baby is born.^[6][11]

Women planning a future pregnancy should inform their GP or midwife at the earliest opportunity, as they will need early screening for GDM in subsequent pregnancies. Optimising weight and glucose control before conception can help reduce the risk of recurrence and improve outcomes for both mother and baby.

Frequently Asked Questions