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Why a healthy weight in pregnancy matters

What is the Body Mass Index? How will it affect your care?

The body mass index (BMI) is a measure that uses your height and weight to work out if your weight is healthy. Whatever their BMI most women will experience a straightforward pregnancy and have a healthy baby, however, being overweight or underweight increases your risk of pregnancy related conditions that may affect your health or your baby.

At your booking visit your Midwife will calculate your body mass index (BMI) using your weight and your height (it is your weight in kilos divided by your height in metres squared).

Using the BMI table, your Midwife will recommend a plan of care and record this in your notes.

Your BMI

Your BMI at booking
Your Weight Status
Your Recommended Lead Professional
Below 18.5 Underweight Consultant
18.5 – 24.9 Healthy Weight Midwife
25.0 – 29.9 Overweight Midwife
30.0 – 34.9 Obesity Class 1 Midwife
35.0 – 39.9 Obesity Class 2 Consultant
Over 40 Obesity Class 3 Consultant and will need anaesthetic review


If your BMI at booking is greater than 30, we recommend that you take an increased daily dose of folic acid (5mg) and aim to gain no more than 9kgs during the pregnancy. Folic acid is available on prescription (if possible start taking doses before conception).

If your BMI is over 35 at booking, you are at somewhat greater risk of developing complications in pregnancy than the general population. You will be offered specialist advice and guidance. If you are expecting your first baby it will be recommended you give birth in the Consultant Unit at Stoke Mandeville Hospital rather than at home or in a Midwife Led Unit. If it is your second or subsequent baby, your pregnancy is straightforward, and you have given birth before vaginally, you may be advised to plan your baby’s birth at the Aylesbury Birth Centre.

If your BMI is 40 or above at any point in your pregnancy you will be offered an appointment with a Consultant Anaesthetist. He/she will offer you an examination to assess any risks for anaesthesia and discuss your options with you. This visit will include an assessment of any special equipment required e.g. a special mattress, as well as an assessment of your skin condition so that we can prevent the development of pressure sores and help you plan for a safer birth.

If your BMI is less than 18.5 at booking, most mothers will have a healthy pregnancy and baby. You are at slightly increased risk however, of having a miscarriage, premature birth or a baby of low birthweight and extra ultrasound scans are sometimes offered in these circumstances.

What are the complications and difficulties I am at risk of in pregnancy?

You are at greater risk of developing high blood pressure. Your Midwife or doctor will check this at all antenatal visits, and you may be offered more frequent appointments later in your pregnancy in order to monitor your blood pressure more closely. You may be advised to take aspirin (150 milligrams) daily from the 12th week of your pregnancy.

If your BMI is over 30 at booking, your risk of developing diabetes in pregnancy is three times greater than with a BMI less than 25. A glucose tolerance test (GTT) or equivalent will be recommended at 28 weeks gestation to assess if you have developed this condition.

A high BMI also puts you are at greater risk of developing a blood clot (thrombosis) which can lead to serious complications. Depending on other risk factors, you may be offered daily injections to prevent blood clots during your pregnancy and/or after the birth of your baby.

There can be difficulty assessing your baby’s growth and you may need to have additional scans.

What are the risks for my baby?

The overall likelihood of a miscarriage in early pregnancy is 1 in 5 (20%), but if you have a BMI of 30 or above, your risk increases to 1 in 4 (25%). If you are overweight before pregnancy or in early pregnancy, this can affect the way your baby develops in your womb (uterus). Overall, around 1 in 1000 babies in the UK are born with problems with the development of the baby’s skull and spine (neural tube defects), but if your BMI is 30 or above, this risk is nearly doubled (2 in 1000).

If you are overweight, you are more likely to have a baby weighing more than 4kg, which increases the risk of complications for you and your baby during birth. If your BMI is 30 or above, your risk is doubled from 7 in 100 to 14 in 100 compared with women with a BMI of between 20 and 30. These risks will be discussed with you in greater detail during your consultant appointment.

The overall likelihood of stillbirth in the UK is 1 in every 200 births. If you have a BMI of 30 or above, this risk increases to 1 in every 100 births.

If you have a high BMI during pregnancy, you may need additional ultrasound scans to check your baby’s development, growth and position. Your baby’s growth is normally monitored during pregnancy using a tape measure to record the size of the uterus. If your BMI is more than 35 then it may be difficult to be accurate with a tape measure so your healthcare professional may request additional ultrasound scans.

All women in the UK are offered an ultrasound scan at around 20 weeks to look for structural problems that your baby may have. This scan is less accurate at picking up problems if you have a high BMI.

How can I help myself during pregnancy?

Healthy eating – it is important to be careful with the size of portion you eat as well as snacks and how often you eat. We advise you to restrict the amount of starchy carbohydrates you eat (e.g. bread, potatoes, rice, pasta and products made from them) and cut down on high calorie fatty and sugary food including sugary drinks. Keep a food diary—keeping an accurate account of your daily food intake can help you avoid excessive calorie intake.
You do not need extra calories for the first two thirds of pregnancy, it is a myth that you are eating for two. It is only in the last 12 weeks that you need an extra 200 kilocalories a day (this equates to less than two slices of unbuttered bread).

You can access free dietary support and advice to help you during your pregnancy from Live Well Stay Well by registering or by asking your midwife to refer you.

Exercise – we recommend 30 minutes of moderate exercise a day e.g. brisk walking or swimming. However, if you are not used to exercise then start with 15 minutes of continuous exercise 3 times a week and gradually increase to 30 minutes per day.

Planning for labour and birth if you have a BMI over 35

Your BMI will be recalculated when you are 36 weeks pregnant, taking into account your expected weight gain in pregnancy and your midwife will help you with your birth plan, in addition to the measures noted above:

  • We will encourage you to be active and mobile in labour.
  • Monitoring your baby’s heartbeat in labour can be difficult. If we need to continuously monitor your baby’s heartbeat it may be necessary to attach a small clip to your baby’s head (foetal scalp electrode) to make the monitoring more accurate.
  • If you plan to use the birthing pool it is essential that you can enter and exit the pool unaided and that the Midwife can easily listen to your baby’s heartbeat.
  • If this is your first baby, you are at increased risk of complications in labour and a greater risk of requiring a Caesarean section.
  • During labour you might be given a tablet to help reduce the acid contents in your stomach.
  • You may be discouraged from eating in labour should an emergency caesarean section become necessary; however, this recommendation is made on an individual basis.
  • There is an increased risk of bleeding after birth. We would recommend an injection to aid the delivery of the placenta. We may also recommend a drip for up to 4 hours after the birth to ensure the risk of bleeding stays low.

After your baby is born

  • We will encourage early mobilisation and may advise the use of compression stockings to reduce your risk of blood clots, especially if you have had a Caesarean section. You may be advised to have daily injections to prevent blood clots after the birth.
  • Breastfeeding has numerous health benefits to you and your baby and has been shown to reduce the likelihood of your child becoming obese. Support with breastfeeding will be offered from birth and continue when you return home if you have a hospital birth.
  • Following birth is an ideal time to consider weight reduction. If you would like help and advice on weight reduction, ask your GP who may be able to refer you to a Dietician or support group.

Please remember that this leaflet is intended as general information only. We aim to make the information as up to date and accurate as possible, but please note that it is subject to change. Please therefore always check specific advice on any concerns you may have with your doctor.

How can you help reduce healthcare associated infections?

Infection prevention & control is important to the well-being of our patients and for that reason we have infection prevention and control procedures in place. Keeping your hands clean and wearing a mask is an effective way of preventing the spread of infections. Please follow our infection prevention and control guidelines when visiting our healthcare sites. Further information is available on our website.

Patient Advice Sheet

If you would like a copy of this information on audiotape, in large print or translated, please call the Patient Advice Liaison Service on 01296 831120 or email