Body weight is an important consideration for health outcomes. BMI is used to assess if a woman is overweight, underweight, or at a healthy weight for her height.
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Why is weight important?
A woman’s weight prior to becoming pregnant is an important factor in health outcomes, weight gain during pregnancy, and other nutritional considerations.
Women who are overweight are at an increased risk for gestational diabetes, hypertension, and preeclampsia [1]. The risk of neural tube defects (NTD) is doubled for women who are obese compared to those who are at a normal weight and chances of a cesarean section are also increased.
On the flip side, women who are underweight are at risk of having babies who are too small, which can pose a risk to their growth and development [1].
How do I know if I’m at a normal weight?
Body mass index (BMI) is the standard indicator used to categorize individuals as underweight, normal weight, overweight, or obese. The following chart shows the BMI ranges and corresponding categories [2].
| BMI | Weight Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Normal or healthy weight |
| 25.0 - 29.9 | Overweight |
| 30.0 and Above | Obese |
How do I calculate my BMI?
You can calculate your BMI using the equation below or simply plug in your height and weight into this calculator.
Imperial: BMI = (Weight in pounds) / (Height in inches)² × 703
Metric: BMI = (Weight in kilograms) / (Height in meters)²
For example, a woman who is 5′ 3″ tall and 130 pounds has a BMI of 23, which is considered normal. At 160 pounds, that same woman would have a BMI of 28 and be considered overweight.
What is BMI used for?
BMI is not a definitive marker of health, it is simply a screening tool for health guidance. It is well known that body composition (amount of fat and muscle) can influence an individual’s weight. However, unless you are participating in bodybuilding or competitive athletics, it’s likely that your BMI range is an accurate indicator, imperfect as it may be.
Your BMI does not determine your health nor the health of your baby. Of course, being outside of the normal BMI range does not mean that you will have pregnancy complications. And equally, being at a normal BMI does not mean that you won’t experience any health challenges.
What should I do if I’m currently overweight?
You are not alone, nearly two-thirds of women in the US of childbearing age are overweight or obese. To minimize risk to both mom and baby, the National Academy of Medicine (NAM) recommends that women achieve a normal BMI prior to getting pregnant and stay within the suggested weight gain range [3].
To decrease the risk of neural tube defects, women with BMIs over 29.9 should increase supplementation of folate above the generally recommended amount. The Royal College of Obstetricians and Gynaecologists (formal health organization of the UK) recommends 5 mg of supplemental folate daily starting at least a month before getting pregnant [4]. NAM has not published its recommendation.
Another risk associated with high BMI is a vitamin D deficiency since it is held up in fat cells and therefore less bioavailable. Different health organization have varying recommendations on additional supplementation of vitamin D, however, official guidelines have not been published [5].
If you are overweight or obese, it is a good idea to bring up folate and vitamin D concerns with your health provider.
Summary
- The best thing you can do prior to becoming pregnant is to be at a healthy weight.
- Calculate your BMI and use that as a guideline, not a determinant, for your nutritional considerations.
- If you are overweight or obese, be aware of increased risks during pregnancy, including folate and vitamin D deficiencies.
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