BMI – Body Mass Index

What is BMI?

BMI, short for Body Mass Index, is a way of assessing your weight relative to your height.

You calculate it 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:

BMI graphic showing 'underweight', 'normal', 'overweight' and 'obese'

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.

Obesity & BMI

‘Obesity’ – is officially classified as someone with a BMI greater than 30. Many people detest the term, and I’m not a big fan of it either, but given that the medical literature uses the label frequently, on a science-following site like Big Birthas, it’s all but impossible to avoid.

Personally, I find the terms ‘Healthy Weight’ (looking at you, NICE!) and ‘Morbidly Obese’ (seriously, NHSIC?) very loaded, and not at all helpful. Health, both mental AND physical, is about a lot more than numbers on a scale.

And I don’t want to get sidetracked, but BMI was never intended to be used at an individual level – it was devised as a population level statistical analysis tool, and is loaded with problematic (particularly, age, sex and race) biases. But science needs a handy way to put people into boxes in order to look at data, so here we are.

We are beginning to see some pushback against this one-BMI-fits-all strategy, with some researchers and government departments advocating for different bandings for different ethnic groups – so if you are of Black Caribbean, South Asian, Chinese, or Arab descent living in England, you may start to find people referring to you as overweight or obese even if your BMI is lower than the banding in the table below.

NICE and NHSIC classifications of BMI

Body Mass IndexNICENHSIC
less than 18.5Unhealthy weightUnderweight
18.5 – 24.9Healthy weightNormal
25.0 – 29.9OverweightOverweight
30.0 – 34.9Obesity IObese I
35.0 – 39.9Obesity IIObese II
40 or greaterObesity IIIMorbidly Obese

According to Public Health England analysis of maternity data in 2019, around one in five pregnant women and birthing people in the UK have a BMI over 30.


Your community midwife will usually want to weigh you and measure your height at your first appointment, who will calculate your BMI and write this on your pregnancy notes.

This will very likely happen all over again at your first appointment at the hospital. 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, as the guidance states it is not helpful:

Do not weigh women repeatedly during pregnancy as a matter of routine. Only weigh again if clinical management can be influenced or if nutrition is a concern

Weight management before, during and after pregnancy – NHS

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.

Why calculate it at all?

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 and birthing people are classified as ‘obese’.

You will probably be told that those 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…

and the list goes on. Many have left these appointments in tears, fearing the worst. The trouble is, the professionals don’t tend to say by how much that risk has increased. HighER is not necessarily ‘high’!

The majority of bigger pregnant women have normal births!

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 pregnant women, irrespective of size, 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.

Some studies have shown positives to being overweight, but no-one ever mentions those!!

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 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 higher BMIs in Sweden.

BMI 35+

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. (all of which is your choice to accept or decline).

BMI 40+

If your BMI is 40 or greater (which puts you in the ‘morbidly obese’ or obesity III class, ugh) 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.

Just to be clear, you actually don’t have to accept any of these offers of extra care.

You can’t do much about the denial of access to facilities on their property (their turf, their rules) but the rest are ‘offers’. They may be presented to you as a foregone conclusion. They may be phrased as ‘our policy is…’ but however these options are put to you, they are just that; options for you to consider. You’re perfectly at liberty to listen to the midwife or other professional tell you they’re going to refer you to X, Y and Z, and then say “Yes, now, about your offer to X… Yeah, I think I’ll pass on that, thanks.”

Find out how this applies to you locally

It’s worth having a looking at the particular policies of the hospital you are thinking of using beforehand. This way you are forewarned and forearmed – some Trust policies are listed here as examples, but they’re out of date now. Submit your own FOI request; instructions 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 our treatment was solely with regard to our individual circumstances, rather then merely because of a line on a graph. Sadly many hospitals pigeonhole us based solely on our BMI, without regard to overall health and activity levels, previous birth experience etc.

The upshot of this is that the pregnancies, labours and births of bigger people can have a tendency to become overmedicalised. While some of us do need the extra help (as do birthers of all sizes!), and we are extremely fortunate that it is available, the majority of us will have normal, uncomplicated pregnancies and births! This is not reflected in policies and attitudes. We’re not treated normally, and are in some cases unfairly being denied access to facilities merely on the basis of 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. Neither tells you about someone’s sleep, stress levels, mental health, dietary intake, activity levels or anything much.

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.

Lastly, check that the health professional who records your BMI calculated it 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.

Remember, BMI is only one VERY LIMITED way of assessing your health. It does not define you, your pregnancy, or what is likely to happen in it.


You can find links to all BigBirthas Research and Guidance Reference Citations here

Obesity: guidance on the prevention, identification, assessment and management of overweight and obesity in adults and children

Statistics on Obesity, Physical Activity and Diet: England, 2013

Health of women before and during pregnancy: health behaviours, risk factors and inequalities An updated analysis of the maternity services dataset antenatal booking data

Weight management before during and after pregnancy

Linear association between maternal pre-pregnancy body mass index and risk of caesarean section in term deliveries

Effect of Body Mass Index on pregnancy outcomes in nulliparous women delivering singleton babies

Maternal obesity and pregnancy outcome: a study of 287 213 pregnancies in London

Fetal macrosomia- a continuing obstetric challenge

Transition from overweight to obesity worsens pregnancy outcome in a BMI-dependent manner

Outcome of pregnancy in a woman with an increased body mass index

The unethical use of BMI in contemporary general practice

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