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Why being overweight in pregnancy matters

Read our guide below on why being overweight in pregnancy matters and why we measure your BMI.

You can also download a PDF version of this patient information by following the link on the right.

What is the Body Mass Index (BMI)? How will it affect my care?

When we book your visit, your midwife will calculate your body mass index (BMI) using your weight (kg) and your height (cm).

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

Your BMI  Your weight status Your recommended lead professional for further care
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.

If your BMI is over 35 when we re-weigh you at 36 weeks, we’ll transfer your care to a consultant.

35.0 – 39.9 Obesity class 2 Consultant.

If this is your second or subsequent baby and your BMI is less than 40 when we weigh you at 36 weeks,  we may transfer your care to a midwife.

Over 40 Obesity Class 3 Consultant.  You’ll also need anaesthetic review.

Your BMI

If your BMI is more than 30 when we book your visit, we recommend you buy a Vitamin D preparation containing 10mcg (or 400 international units). Take one tablet every day.

We recommend this because you’re at more risk of Vitamin D deficiency which can affect you and your baby.

Find out more about Vitamin D in pregnancy.

If your BMI is over 35

You’re at more risk of developing pregnancy complications and we’ll give you specialist advice and guidance.

If you’re expecting your first baby, we recommend you give birth in the Consultant Unit at Stoke Mandeville Hospital instead of at home or in a Midwife Led Unit.

If it’s your second or subsequent baby, your pregnancy is straightforward and you have given birth before vaginally, we may support a planned birth at the Aylesbury Birth Centre.

If your BMI is over 40

If your BMI is 40 or above at any point in your pregnancy, we’ll offer you an  appointment with a consultant anaesthetist. They’ll offer you an examination to assess any risks for anaesthesia and discuss your options. We’ll also assess you for any special equipment, for example, a special mattress, and an assessment of your skin condition so that we can prevent pressure sores and help you plan for a safer birth.

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

You’re at greater risk of developing high blood pressure. Your midwife or doctor will check this at all antenatal visits. Sometimes you may need additional blood and urine tests. We may recommended taking 75mg of aspirin daily from the 12th week of your pregnancy.

If your BMI is over 30 at booking, you have a greater risk of developing diabetes in pregnancy. We recommend a glucose tolerance test (GTT) at 28 weeks to assess if you have diabetes.

You’re at greater risk of developing a blood clot (thrombosis) which can lead to serious complications. Depending on other risk factors, we may give you 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.

How can I help myself during pregnancy?

Healthy eating

It’s important to monitor meal portion sizes, as well as snacks and how often you eat. We recommend that you:

  • restrict the amount of starchy carbohydrates you eat, for example, bread, potatoes, rice, pasta and products made from them
  • cut down on high calorie fatty and sugary food and sugary drinks
  • keep a food diary to help you avoid excessive calorie intake.

You do not need extra calories for the first two thirds of pregnancy. It’s only in the last 12 weeks that you need an extra 200kcal per day. This equates to less than 2 slices of non-buttered bread.

If your BMI is more than 30, talk to your midwife about referral for extra dietary advice and support. We can provide this for 12 weeks during pregnancy, free of charge.


We recommend 30 minutes of moderate exercise each day, for example, brisk walking or swimming. If you’re not used to exercise, start with 15 minutes of continuous exercise 3 times a week. Gradually increase it to 30 minutes each day.

Planning for labour and birth with a BMI over 30

We recalculate your BMI when you’re 36 weeks pregnant.

Your midwife will help you with your birth plan. We promote normal birth and encourage you to be active and mobile in labour.

Monitoring your baby’s heart beat in labour can be difficult. If we need to continuously monitor your baby’s heartbeat we may need to attach a small slip to your baby’s head (fetal scalp electrode) to make monitoring more accurate.

What to expect

This may include:

  • a caesarean section if this is your first baby due to an increased risk of complications – we may discourage you from eating during labour if you need an emergency c-section
  • a tablet during labour to help reduce the acid contents in your stomach
  • increased risk of bleeding after birth. We recommend an injection to help with the delivery and give you a drip to reduce the risk of bleeding.

If you plan to use the water pool you must enter and exit the pool unaided so that the midwife can easily listen to your baby’s heartbeat.

After the birth

We encourage early movement and may tell you to use compression stockings to reduce your risk of blood clots, especially if you have had a caesarean. You may need to have daily injections to prevent blood clots after the birth.


It has numerous health benefits to you and your baby and can reduce the likelihood of your child becoming obese. If you need support please speak to your midwife.

Weight reduction

It’s now an ideal time to consider weight reduction. If you’d like help and advice, ask your GP who may be refer you to a dietitian.