BMI During Pregnancy – Is It An Accurate Measurement Of Health?

BMI During Pregnancy – Is It An Accurate Measurement Of Health?

Whether you are planning to become pregnant, or you're about to have your third baby, you’re probably worried about weight gain.

You might have been told your weight will affect your risk status during pregnancy and birth.

No doubt you’ve been told having a high body mass index (BMI) during pregnancy leads to an increased risk of gestational diabetes, hypertensive disorders of pregnancy, and a big baby.

Having a BMI over the normal range can mean changes to your birth care, even late in your pregnancy.

BMI During Pregnancy

But does BMI really indicate how healthy you are?

A recent study published in the International Journal of Obesity suggests using BMI to gauge health is causing millions of people to be incorrectly categorised as unhealthy.

What Is BMI?

Body mass index, or BMI, has been used for over 100 years to help health professionals decide whether a person is overweight or underweight.

The BMI was introduced in the 1830s by Belgian astronomer, mathematician, statistician and sociologist, Lambert Adolphe Jacques Quetelet.

He came up with a simple mathematical formula to provide a simple and easy way to measure obesity in the general population, and to help determine how government resources would be allocated.

The BMI wasn’t universally used as a measurement until the 1980s. In 1998, at the height of the low-fat diet craze, the National Institutes of Health approved BMI as the standard measurement of health.

Body mass index (BMI) is a measure of body fat, based your weight, in relation to your height. The figure is reached by dividing your weight by your height squared: BMI = weight (kg) / height (cm) 2

According to most criteria accepted worldwide:

  • A BMI of 18.49 or below means a person is underweight
  • A BMI of 18.5 to 24.99 signals normal weight
  • A BMI of 25 to 29.99 means a person is overweight
  • A BMI of 30 or more means a person is obese

Is BMI Accurate?

The study quoted above found BMI wasn’t an accurate measurement of health.

Using data from the Centers for Disease Control and Prevention’s National Health and Nutrition Examination Survey, the researchers looked at the link between BMI and health markers. They included factors such as glucose, cholesterol, and blood pressure.

The study showed nearly half of all those considered overweight according to BMI were in fact healthy. About 19 million people with a BMI in the obese range were also metabolically healthy.

The researchers also found over 30% of people in the normal or healthy BMI range were metabolically unhealthy.

At the time Quetelet came up with the BMI formula, he didn’t have access to calculators or computers, so his system was quite simple.

Today, we have technology to add more information to the mix. People are naturally of different shapes and sizes. Muscle is denser than fat, and weighs about 18% more. BMI doesn’t take these factors into account.

BMI And Pregnancy

Maternity health experts caution women, telling them to make sure they’re within a healthy weight range (with a BMI of 18.5 to 24.99) before falling pregnant. This increases their chance of a low risk pregnancy and birth.

Women who, according to their BMI, are considered overweight or obese are encouraged to lose weight before becoming pregnant.

According to the American College of Obstetricians and Gynecologists (ACOG), there are significant risks associated with obesity and pregnancy.

Obesity increases the risk of miscarriage, and has been linked to an increased risk of gestational hypertension, preeclampsia, and gestational diabetes.

As a woman’s BMI increases, so do her chances of having interventions during labour, and a c-section birth.

Recent research, however, shows this might not just be about being overweight. It might also have a lot to do with care providers’ attitude towards the care of obese women.

What Does BMI Mean During Pregnancy?

If BMI isn’t a good indicator of health, and if people are potentially being classed as obese when they aren’t, where does this leave pregnant women?

The current mindset of most care providers is women who are outside the normal BMI range aren’t a healthy weight and will therefore be treated as though they are high risk.

This is frustrating for pregnant women who are healthy and have no other risk factors; it doesn’t take their overall health into consideration.

Women who are healthy, with no or low risk factors for complications, might experience a biased attitude to their pregnancy and birth care, because judgements are based on an outdated and inaccurate measurement of health.

Is There A Better Way To Measure?

Research shows it’s not about how much you weigh, but rather where you’re carrying excess weight; this is the most significant factor with regard to your health.

BMI doesn’t take into consideration where fat is being stored on your body. A measurement of your waist circumference, on the other hand, gives a better indication of the amount of fat around vital organs such as your heart, liver, pancreas and kidneys.

This is important because fat around your organs releases hormones, which can affect blood pressure, cholesterol and blood sugar levels. In turn, this raises your risk of diabetes, heart disease, stroke etc.

Research has shown calculating a person's waist-to-height ratio is the most accurate and efficient way of identifying whether or not they are at risk of obesity.

The ratio is determined by dividing your waist circumference by your height, both measured in the same units. Your waist measurement should be less than half your weight: e.g. if you're 167cms (66 inches) in height, your waist should be 83cm (33 inches) or less.

What If My BMI Is High?

If you are planning to become pregnant, it is a good idea to talk to your care provider about your overall health.

This article has outlined the reasons why BMI is not an accurate measurement of health. You can calculate your waist-to-height ratio and discuss with your care provider any potential health problems you might face during pregnancy.

Eating a nutritious diet and doing some form of exercise are always steps in the right direction towards having a problem free pregnancy.

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Sam McCulloch Dip CBEd CONTRIBUTOR

Sam McCulloch enjoys talking so much about birth that she decided to become a birth educator and doula, supporting parents in making informed choices about their birth experience. In her spare time she watches Downton Abbey and has numerous creative projects on the go. She is mother to three beautiful little humans.


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