Body Mass Index (BMI)

What is it?

Body Mass Index is a way of assessing your weight relative to your height.

It is calculated by dividing your weight in kilograms by the square of your height in metres (kg/m²).

So, if your height is 5 ft 3 in (approximately 1.6 metres) and you weigh 10 stone (approximately 63.5kg) your BMI would be calculated like this:

63.5 divided by 1.6²

= 63.5 / 2.56

= 24.8kg/m²  – so your BMI would be 24.8

There are planty of online calculators like this one: NHS Choices BMI Calculator to make your life easier.

NICE and NHSIC classifications of BMI

Body Mass Index NICE NHSIC
less than 18.5 Unhealthy weight Underweight
18.5 – 24.9 Healthy weight Normal
25.0 – 29.9 Overweight Overweight
30.0 – 34.9 Obesity I Obese I
35.0 – 39.9 Obesity II Obese II
40 or greater Obesity III Morbidly Obese


You will usually be weighed and your height measured at your first appointment with your community midwife who will calculate your BMI and write this on your pregnancy notes.

You will likely be weighed and measured again at your first appointment at the hospital who will also calculate your BMI. The odds of this agreeing with the first measurements are small, and most likely the hospital will overwrite the measurements of your community midwife on your notes, despite the fact that you will probably be further on in your pregnancy and therefore likely to weigh more!

Hopefully you then won’t be measured again, although some hospital trusts like to weigh you shortly before your due date to make sure that the equipment they plan to use is suitable for your weight.


Most hospitals use BMI as a means of identifying patients who may be at additional risk in pregnancy. The NHS estimates that 15-20% of pregnant women are classified as obese.

You will probably be told that women with BMI over 30 have greater risk of:

gestational diabetes
high blood pressure
blood clots
going overdue
induction of labour
instrumental delivery (ventouse or forceps)
caesarean section
large babies (macrosomia)
baby’s shoulders getting stuck (shoulder dystocia)
wound infection
excessive bleeding after the birth (post partum hemorrhage)
still birth
maternal death
problems breastfeeding… the list goes on. Many women have left these appointments in tears, fearing the worst.

While much is made of these risks, and many larger pregnant women feel like their pregnancy is doomed to complications, this is not the case. The majority of obese pregnant women go on to have perfectly normal births.

No matter what impression an overzealous health professional gives you, it is worth keeping in mind that the risks, while greater for overweight women, are still very small. Gestational diabetes, for example, according to one study of 16,000 women, arises in 2.3% of women whose BMI is under 30, and only 9.5% of women whose BMI is over 30. That still leaves more than 90% of overweight women NOT developing GD.

Also, many of the above risks follow one another in what’s known as a cascade or spiral of interventions. For example, going over your due date is likely to lead to obstetricians wanting to induce, induction is known to lead to increased likelihood of instrumental delivery and caesarean section, problems with would healing and so on. If you can avoid the first of the interventions, you are less likely to experience the rest.

The studies that the doctors are oh so happy to refer to when scaring you about complications also have on occasions shown positives to being overweight – for example the Cedergren study on Obesity and Pregnancy outcome (link below), may show an increased rate of pre-eclampsia (3.4% of pregnancies to obese mothers, as opposed to 1.4% in the normal weight group), but the doctors are unlikely to tell you that the same study actually showed a reduction in anal sphincter laceration to obese mothers of 1.9% instead of 2.6%! The same study also showed a reduction in instrumental deliveries for bigger mums, which might be an interesting reflection of more accepting attitudes and practices towards bigger mums in Sweden.

If you have a BMI of 35 or greater you will most likely automatically be referred for consultant-led care, may be ineligible to use midwife-led birthing suites, may be seen more often, be scanned more often, be referred for glucose tolerance testing etc. etc.

If your BMI is 40 or greater (which puts you in the morbidly obese or obesity III class) you may well be subject to further tests, and further restrictions. You may be referred to a dietician, an anaesthetist, be prescribed a blood thinning agent to inject daily and be asked to complete a demeaning form asking you whether you can get up out of a chair or off of a bed by yourself, despite the person asking you the questions having just watched you walk in through the door and sit in a chair perfectly normally. You may be denied access to a hospital birthing pool.

You may want to look into the particular policies of the hospital you are thinking of using so you are forewarned and forearmed about what you are likely to be asked and referred to – Trust policies are listed here. That way, nothing will come as a surprise and you can do your own research about whether you believe the risks being presented are real for your personal circumstances.

It would be nice if women were treated with regard to their individual circumstances, rather then merely because of a line on a graph, but many hospitals at the moment pigeonhole women based solely on their BMI, without regard to their overall health and activity levels, previous birth experience etc.

The upshot of this is that the pregnancies, labours and births of bigger women are becoming overmedicalised. While some women do need the extra help, and we are extremely fortunate that it is available, the majority of overweight pregnant women have normal, uncomplicated pregnancies and births, yet are not being treated normally, and are in some cases unfairly being denied access to facilities merely on the basis of their BMI.

If our pregnancies are otherwise normal, we should not have to fight to be treated as any other pregnant woman.

Things to Consider

BMI is a blunt instrument. It is more useful than weight alone, as it takes height into account, but it is of no practical use when it comes to determining how healthy someone is. Muscle weighs more than fat. Some olympic athletes would register as overweight or obese!

It’s definitely worth checking that the health professional who records your BMI got it absolutely right. Use an online calculator where you can input your weight and height (in metric and imperial measurements) like NHS Choices BMI Calculator. Many hospital trusts use BMI as a gauge for what services you can and can’t use, and if a graph method of calculation is used, this is easily plotted wrongly.

You can refuse to be weighed. Like all aspects of maternity care, it is the job of the professionals to offer, and our job to accept (or not). But this could cause a delay if for any reason you needed anaesthesia, as an accurate weight measurement is necessary for calculation of the drugs needed.


Obesity: guidance on the prevention, identification, assessment and management (1.5 MiB)

Statistics on Obesity, Physical Activity and Diet 2013 (1.5 MiB)

Weight management before, during and after pregnancy (257.2 KiB)

Linear association between maternal BMI and risk of caesarean section (531.0 B)

Effect of Body Mass Index on pregnancy outcomes (259.3 KiB)

Maternal obesity and pregnancy outcome: a study of 287,213 pregnancies in London (216.5 KiB)

Fetal macrosomia - a continuing obstetric challenge (86.3 KiB)

Transition from overweight to obesity worsens pregnancy outcome (12.4 KiB)

Outcome of pregnancy in a woman with an increased BMI (12.5 KiB)

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