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Key takeaways

• Most people with a raised body mass index (BMI) have healthy pregnancies.
• A raised BMI can increase the risk of some pregnancy complications.
• Your care team will offer extra monitoring and support during pregnancy.
• After birth, focus on preventing blood clots and healthy lifestyle choices.

What is a raised BMI and what does it mean for pregnancy?

A measure of your weight in relation to your height is called your body mass index (BMI). It is a standard way for healthcare professionals to check if your weight is in a healthy range for your height. Most people with a high BMI have a straightforward pregnancy and healthy babies.

The BMI ranges are:

• Healthy: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obese: 30 or over

What are the risks of a raised BMI?

However, having a BMI in the overweight or obese range can increase the risk of complications for you and your baby. The higher your BMI, the greater these risks.

People with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background have a higher risk of developing some long-term conditions, such as type 2 diabetes, with a lower BMI.

Potential health considerations during pregnancy

Miscarriage and stillbirth

Miscarriage is common in all pregnancies. Some studies suggest a slightly increased risk of miscarriage with a higher BMI, but most people will still have a healthy pregnancy.

The risk of stillbirth in the UK is low, at around 1 in 200 births. With a BMI over 30, the risk is higher than average, and increases with BMI.

Gestational diabetes

Gestational diabetes is a type of diabetes that can develop during pregnancy. If your BMI is 30 or above, you are three times more likely to develop gestational diabetes, compared with people who have a BMI under 25.

High blood pressure and pre-eclampsia

Having a raised BMI increases your risk of developing high blood pressure. It also increases your risk of pre-eclampsia. People with a BMI of 30 or above are around 2 to 4 times more likely to develop pre-eclampsia than those with a BMI below 25.

Baby with a high birth weight

If you have a raised BMI, you are more likely to have a baby with a high birth weight. This is usually defined as weighing more than 4kg (about 8lb 13oz). Compared with people with a BMI between 20 and 30, this chance doubles from about 7 in 100 to 14 in 100. Having a larger baby can increase the risk of complications during labour and birth for both you and your baby.

Other risks

With a higher BMI there is also a slightly higher chance of problems with the development of the baby's skull and spine (neural tube defects, such as spina bifida). There is also an increased risk of developing a blood clot in a deep vein (deep vein thrombosis, or DVT)

Your care plan during pregnancy and birth

Antenatal care

If your BMI is 30 or more, you will be offered a test to check for gestational diabetes (a glucose tolerance test, or GTT). It is usually done at 24 to 28 weeks of pregnancy. You may be offered an earlier test if you have had gestational diabetes before or have other risk factors.

If the test shows you have gestational diabetes, you will be referred to a specialist. They will provide further testing and treatment if needed.

Your blood pressure and urine will be checked at every antenatal appointment. You may be advised to take low-dose aspirin from 12 weeks of pregnancy if you have additional risk factors for pre-eclampsia. This can help reduce your risk.

If your BMI is raised, you may be offered an appointment with an anaesthetist. This gives you an opportunity to discuss pain relief options for labour and birth, including an epidural. It also helps plan your care in case you need anaesthesia for a caesarean section.

Healthy eating and exercise

You will be given information and advice about healthy eating and physical activity during pregnancy. You may also be referred to a dietitian or a weight management service. It is not recommended to try and lose weight by dieting during pregnancy. However, making healthy changes to your diet may mean you do not gain weight.

If you haven't exercised regularly, you can aim to build up to regular moderate activity, aiming for around 150 minutes per week. If you feel comfortable, you can gradually increase this to 30 minutes on most days. Examples of healthy exercise include swimming, walking, and pregnancy yoga.

Ultrasound scans

All pregnant people in the UK are offered an ultrasound scan at around 20 weeks. This scan checks your baby's development in detail. If your BMI is raised, it can sometimes be harder to obtain clear images of your baby.

During pregnancy, your baby’s growth is normally monitored by measuring your abdomen with a tape measure. If your BMI is more than 35, these measurements may be less accurate. Your midwife or doctor may therefore recommend additional ultrasound scans to monitor your baby's growth and position.

Labour and birth

If your BMI is 30 or more, you will be advised to have your baby in a hospital. This ensures immediate access to specialist care if you or your baby need it.

Prevention of thrombosis

During pregnancy and after birth, you have a slightly higher chance of developing a blood clot in a deep vein (deep vein thrombosis, or DVT). This risk is further increased if you have a higher BMI or other risk factors.

To reduce this risk you may be advised to take blood-thinning injections during pregnancy and for up to 6 weeks after the birth, depending on your risk factors.

To reduce the risk of a blood clot developing:

• Try to be active as soon as you feel comfortable.
• Avoid sitting still for long periods.
• Wear special compression stockings if advised.

Breastfeeding support

How you choose to feed your baby is a very personal decision. There are many benefits of breastfeeding for you and your baby. It is possible to breastfeed whatever your weight. Extra help is available from your healthcare professional and local breastfeeding support organisations if you need it.

Long-term health

It can be helpful to continue with healthy eating and gentle exercise after your baby is born. If you want to lose weight once you have had your baby, you can discuss this with your healthcare professional. Losing weight after your pregnancy can have long-term health benefits for you.