Calculate your BMI

BMI stands for body mass index, and this is a way for your health team to know whether you are a healthy weight for your height.

BMI calculator

Your height

Your weight (before pregnancy)

BMI is a calculation that is used to work out whether you are a healthy weight for your height. It is a number that is reached by dividing your weight in kilos (kg) by your height in metres squared (m2).

This calculation is for use before and after pregnancy, although your midwife may check it early in your pregnancy as well. If you're trying to get pregnant, you may want to check your BMI using the calculator on this page.

At your first antenatal visit, called the 'booking-in visit', your midwife may measure your height and weigh you. Your booking-in visit normally takes place between eight and 12 weeks of pregnancy.

The midwife will work out your BMI using your height and weight and record it in your notes. Generally, people involved in your health care will describe your BMI as follows:

  • 18.5 or lower = underweight
  • Between 18.5 and 24.9 = healthy weight
  • Between 25 and 29.9 = overweight
  • 30 or higher = obese.

Your BMI (using your pre-pregnancy weight) may highlight a need for some extra care and support during your pregnancy.

What happens if my BMI is not in the 'healthy weight' range?

Doctors and midwives need to record which category you come into based on these BMI groups so they can make sure you are given the best possible care during your pregnancy.

Try not to be offended if anyone involved with your health care uses the words 'overweight' or 'obese' to describe your weight. A lot of women don't like these terms but nobody is judging you, these are clinical measurements. The medical experts looking after you during pregnancy use these categories so they can make sure you have the best advice and support to have the healthiest pregnancy for you.

If your BMI is over 30, you are at higher risk of certain conditions such as high blood pressure, gestational diabetes and developing a blood clot. The higher your BMI, the higher the risks are for you. Therefore you will get extra care and support in pregnancy to lessen these risks and give you the best chance of having a healthy baby.

If your BMI is over 30…

Your antenatal care may include:

  • detailed advice about healthy eating, and possibly a referral to a dietitian
  • talking about ways for you to be more active during your pregnancy
  • a possible referral to a specialist antenatal clinic run by a doctor (obstetrician) or a midwife
  • a glucose tolerance test to check for pregnancy diabetes, although you won't be offered this if you've had weight-loss surgery. 
  • extra blood pressure checks
  • a detailed plan for your care during pregnancy, which will help give you the best chance of things going well at the birth.

If your BMI is over 40…

You can also expect:

  • a referral to an obstetric anaesthetist (a doctor who specialises in maternity anaesthetics and pain relief), who will explain how to reduce the risk of problems during your baby's birth if you need a caesarean delivery or other intervention. 
  • an assessment of how you can move about during labour and after your baby is born to help you avoid getting blood clots or sore areas on your body.
  • extra scans during your pregnancy to see how your baby is growing if the midwife can't easily check your bump.

If your BMI is under 18.5 or under…

If your BMI is under 18.5 it suggests that your weight may be too low and midwives will explore possible reasons.

Midwives are trained to ask if anyone has had any issues with eating or eating disorders in the past. Your midwife will ask if you have ever had any issues with an eating disorder, such as anorexia nervosa. Although it may be difficult, it's important to be honest. Extra support, such as counselling from a mental health professional, can help you cope with the challenges around eating healthily during your pregnancy

If depression or any other mental health condition is affecting your weight, and your ability to gain a healthy amount, your health care team may recommend safe medication for you to take.

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 the specialist who is advising you will discuss with you what your target weight gain in pregnancy should be. This will be based on a number of factors, including your pre-pregnancy weight. They might also ask to weigh you regularly during your pregnancy to check on your progress.

Read more

Sources

  1. CMACE/RCOG (2010)  Joint Guideline Management of Women with Obesity in Pregnancyhttps://www.rcog.org.uk/globalassets/documents/guidelines/cmacercogjointguidelinemanagementwomenobesitypregnancya.pdf
  2. NICE (2010)  Dietary interventions and physical activity interventions for weight management before, during and after pregnancy, Public health guidance 27, 2010
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Last reviewed on February 1st, 2015. Next review date February 1st, 2018.

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