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 pregnant women your BMI calculation will be based on your weight before pregnancy.
Your midwife may measure your height and weigh you 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.
Once you work out your BMI, this is the scale:
- Less than 18.5 = underweight
- 18.5 to 24.9 = healthy weight
- 25 to 29.9 = overweight
- 30 to 39.9 = obese
- 40 = severely obese
Try not to be offended if anyone involved with your health care uses one of these terms to describe your weight. The medical experts looking after you during pregnancy may need to use them so they can make sure you have the best advice and support to help you have a healthy pregnancy.
What happens if my BMI is not in the 'healthy weight' range?
If you have a high BMI (over 25) before pregnancy or in early pregnancy, this can affect your health and how your baby develops. The higher your BMI the higher the risk.
These risks include:
This can be difficult to read but you will have extra care throughout your pregnancy to minimise these risks and have a healthier pregnancy.
If your BMI is over 30
If you have a BMI over 30 you should be referred to a dietitian for an assessment and advice about healthy eating and exercise.
You will also have:
- extra ultrasound scans to check the baby’s development
- blood pressure tests at every antenatal appointment
- a gestational diabetes test between 24-28 weeks (if your BMI is over 40 you may have this test earlier)
- a risk assessment for thrombosis (blood clots in your legs or lungs) at your first antenatal appointment, which will then be monitored throughout your pregnancy.
You may also need to have injections of low molecular weight heparin to reduce your risk of blood clots.
If your BMI is over 40
If you have a BMI over 40 there is a higher risk of complications during labour or birth. These include:
- a longer labour
- premature birth (when a baby is born before 37 weeks)
- an emergency caesarean birth
- a more difficult operation if you need a caesarean section and a higher risk of complications afterward, for example a wound infection or blood clot
- anaesthetic complications, especially with general anaesthesia
- heavy bleeding after birth (postpartum haemorrhage) or at the time of caesarean section
Because of these risks your obstetrician and/or midwife will talk to you about the safest way and place for you to give birth. You should also be referred to an anesthetist (a doctor who specialises in pain relief) to talk about pain relief during labour.
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.
Find out more about being overweight in pregnancy.
If your BMI is under 18.5 or under
If your BMI was less than 18.5 before you became pregnant, you may be advised to gain more weight than someone who is in the normal range.
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.
There are many reasons for having a low BMI. For example, there might be a medical reason, 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). It can be very difficult, but it’s important to tell your midwife or doctor if this is the case. They will be able to organise extra care during your pregnancy so you and your baby can stay healthy.
Find out more about being underweight in pregnancy.