Calculate your BMI

BMI stands for body mass index. It’s important to know what your BMI is as you go into pregnancy because if you are in the higher or lower ranges you may get extra care.

This information is for women and birthing people who are pregnant. If you are planning a pregnancy, read more relevant information here.

On this page

What is BMI? 

BMI ranges

What if I have a high BMI during pregnancy?

Managing the risks of a high BMI while pregnant

Labour and giving birth when overweight

What happens if I have a low BMI during pregnancy?

Managing the risks of a low BMI during pregnancy

What is BMI?

Your BMI is a measure that uses your height and weight to work out if your weight is in a healthy range for your height. For pregnant women your BMI calculation will be based on your weight before pregnancy.  

BMI is not a perfect way to measure your weight. This is because it measures whether someone is carrying too much weight, rather than too much fat. For instance, people who are very muscular may have a high BMI without much fat.  

However, for most people, BMI is still a useful way of measuring their weight range.

Having a high or low BMI may increase the risk of complications to you and your baby, but most people will have a straightforward pregnancy. Calculating your BMI means your midwife can offer you the right care and you can work together to reduce any risks.  

BMI ranges

Your midwife may measure your height and weight to work out your BMI at your first antenatal visit (the booking appointment). This will help them find out if you need some extra care during your pregnancy.  

Your midwife may let you know what your BMI range is. You can also use our calculator above to work out your BMI. Use your weight before pregnancy.

For adults of White heritage, if your BMI is:  

• Less than 18.5 = underweight range
• 18.5 to 24.9 = healthy weight range
• 25 to 29.9 = overweight range
• 30 to 39.9 = obese range
• 40 or above = severely obese range.

The BMI healthy weight range is different for different ethnicities.

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

Because of this, people with these origins should use a different threshold to measure BMI.

If you are from one of these backgrounds, if your BMI is:

• less than 18.5 = underweight range
• 18.5 to 22.9 = healthy weight range
• 23 to 27.4 = overweight range
• 27.5 or above = obese range.

These ranges may mean that people from these ethnic backgrounds get help with their weight earlier than people from White backgrounds.

What if I have a high BMI during pregnancy?

Having a high BMI can increase your risk of some complications. The higher your BMI the higher the risks.

These risks include:

• high blood pressure and pre-eclampsia
• thrombosis (blood clots)
• gestational diabetes (diabetes in pregnancy)
• having a baby with a high birth weight  
• a higher chance of neural tube defects (problems with the development of the baby’s skull and spine).

If you have a BMI of 30 or above, the risk of stillbirth and miscarriage also increases.  

In the UK, the risk of stillbirth is low. It happens in around 1 in every 200 births (0.5%). If your BMI is over 30, the risk goes up to 2 in every 200 births (1%).  

Sadly, early miscarriages are common in pregnancy, whatever your weight. The overall risk of miscarriage in early pregnancy is 10 to 20 out of every 100 births (20%). However, if your BMI is 30 or above, your risk slightly increases to 25 in 100 (25%).

These risks can be tough to read, but it’s important to remember that these risks are still small. Your healthcare team will monitor your pregnancy carefully, and you and your team can work together to reduce these risks.  Read more about risks, benefits, consent and your rights in pregnancy.

Managing the risks of a high BMI while pregnant

Even if you have a high BMI, dieting is not advised as you might cut out nutrients that your baby needs. The best way to protect you and your baby’s health is to go to all of your appointments and follow the advice of your health professionals.  

If you have a BMI over 30 at your antenatal booking visit, you may be offered consultant-led antenatal care. Your healthcare professional will discuss with you any risks for you and your baby, as well as how these can be reduced.

This may involve:  

• taking a higher dose of folic acid until 12 weeks
• taking aspirin from 12 weeks if you have a BMI of 35 or over and any of the following:
  • this is your first pregnancy
  • this is a multiple pregnancy
  • you are age 40 or above
  • you or a close relative had pre-eclampsia
• following advice about healthy eating, exercise, and supplements  
• having extra scans and tests to check your health and the baby’s development
• taking medications to reduce your risk of certain conditions
• having an oral glucose tolerance test (OGTT) between 24 and 28 weeks of pregnancy to check for gestational diabetes (or in the first trimester if you had it before).

Labour and giving birth when overweight

If you have a high BMI there is an increased risk of complications during labour, particularly if your BMI is 40 or above. These risks include:  

• premature birth (baby born before 37 weeks)
• shoulder dystocia  (when one of the baby’s shoulders becomes stuck behind the mother’s pubic bone during a vaginal birth)
• the need for an emergency caesarean section (c-section)
• heavy bleeding after a vaginal birth  
• more complications during and after a c-section.                                       

Being affected by obesity does not always mean you will need a caesarean section but you may be more likely to have one. Find out more about c-section advice for overweight women.  

During your pregnancy, your doctor or midwife will discuss the safest birth plan options with you and ask you about your preferences. This may involve discussing where and how to give birth, as well the best pain relief during labour. You may be advised to give birth in a hospital with quicker access to medical support. Ultimately, how and where you give birth is your decision. Read more about making decisions in pregnancy and risks, benefit, consent and your rights.

To learn more about managing the risks of a high BMI while pregnant, read our advice on being overweight during pregnancy.

What happens if I have a low BMI during pregnancy?

If your BMI was less than 18.5 before you became pregnant, you may need some extra care and support throughout pregnancy. This is because you and your baby may have an increased risk of: 

• premature birth (when your baby is born before 37 weeks)
• having a baby with a low birth weight  
• fetal growth restriction  
• miscarriage  
• problems with your baby’s development.  

It may be hard to read about these risks, but it’s important to remember that most people who are underweight will have a healthy pregnancy and baby. There are also things that you and your healthcare team can do to help reduce these risks and have a healthy pregnancy.  

Managing the risks of a low BMI during pregnancy

Your midwife may refer you to the hospital antenatal clinic for extra scans to check on your baby's growth and development. You may also be referred to a dietitian to help you work on ways to eat well and gain a healthy amount of weight.

Your midwife or specialist will discuss with you what your target weight gain in pregnancy should be. This will be based on several factors, including your pre-pregnancy weight. Your midwife may also give you specific advice and support based on the reason behind your low BMI.  

There are many reasons for having a low BMI. For example there might be a medical reason for your low weight, such as an overactive thyroid. If you think this might be the case, talk to your midwife or GP.

Some women may have an eating disorder (or have had one in the past). An eating disorder can sometimes make it very difficult to put weight on.  

It can feel challenging to talk to your midwife or doctor about this, but it is important. They can support you and organise extra care during your pregnancy so you and your baby can stay healthy. 

Find out more about being underweight in pregnancy.