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Is BMI (Body Mass Index) a Weight Gain Indicator in Pregnancy?

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Rijk de Wet
Last updated: Jan 18, 2024

Is BMI relevant in pregnancy? Does weight gain have anything to do with our health? How can we improve our fertility? We'll answer these and many other questions in the article below. ⤵️

💡 This article is a part of a bigger series, based on our pregnancy weight gain calculator.

What is BMI?

Body mass index (BMI) is calculated with a simple equation that uses both height and weight of a person:

BMI=weight [kg](height [m])2\text{BMI} = \frac{ \text{weight [kg]} }{ (\text{height [m]})^2 }

We use it to assess the weight and proportions of a given person. BMI allows us to determine whether a person is underweight, within normal range, overweight, or obese. The index is easy to use and widely applied in almost all medical sciences.

However, it does come with a few limitations:

  • BMI cannot be used for pregnant women or people with developed muscle tissue. The additional weight in both groups comes from different processes than simply putting on weight.
  • BMI should be modified for people with specific disabilities, e.g., missing a limb.
  • BMI in children shouldn't be used directly, but only via percentiles (try the child BMI percentile calculator).

Is BMI relevant during pregnancy?

Yes, BMI is relevant, but only if we think about the pre-pregnancy BMI. Both your pre-pregnancy weight and height play a vital role in assessing your recommended pregnancy weight gain.

Why is it important, you may ask?

High maternal weight may lead to serious pregnancy complications, such as high blood pressure or gestational diabetes. Overweight and obese women risk having a more traumatic birth; the excessive weight gain may result in a greater occurrence of congenital anomalies and stillbirths.

Women who gain less weight than recommended also bear some risk of adverse pregnancy outcomes, including low birth weight, premature births, and increased mortality of neonates.

The gestational weight gain differs depending on the BMI and ranges between 28–40 lb for underweight women to only 11–20 lb for patients with obesity. For a complete list of recommendations for weight gain, visit the How to maintain pregnancy weight article.

What BMI is safe for pregnancy?

The safest pre-pregnancy body mass index (BMI) is the one within normal range: 18.5–24.9. The maternal weight gain for this BMI group should oscillate between 25–35 pounds (11.3–15.9 kg) for single and 37–54 pounds (16.8–24.5 kg) for twin pregnancy.

Maternal and neonatal outcomes are influenced by both being underweight or obese in pregnancy. Being underweight does slightly less harm than being obese. One thing is certain — women of normal weight have lower chances for many disorders of pregnancy, such as fetal complications or blood-clotting disorders. And, of course, weight gained during pregnancy might also be very challenging to lose.

Talk to your health care provider about possible changes in your lifestyle that could help you regain your healthy weight and improve both your condition and your baby's! 👶

Can BMI affect fertility?

Yes, BMI does affect fertility.

  • Obesity and being overweight affect the function of ovaries and may result in a lower amount of hormones circulating in the bloodstream. Being in these BMI categories may cause problems with getting pregnant and complicate the initial IVF therapy.

  • Extremely low BMI may also harm your potential to conceive — a certain amount of fat tissue is required to produce pregnancy hormones. Women who weigh very little tend to lose their periods and stop ovulating.

Bear in mind that there are tight associations between pre-pregnancy BMIs and a higher risk of complications typical for gestation. Both underweight and overweight women may see worse pregnancy outcomes compared to normal-weight women.

To summarize, keeping a healthy weight gain both before and during pregnancy is one of the best investments you could offer to yourself and your future child!

Does the obese BMI during pregnancy increase health risks?

Yes. High BMI increases health risks in both pregnant and non-pregnant women. The list of possible complications of gaining too much weight is long, and it includes disorders for both mother and child:

  • Hypertensive disorders of pregnancy, including dangerous conditions such as preeclampsia and eclampsia;
  • Gestational diabetes;
  • Congenital anomalies;
  • Delivery of large infants;
  • Miscarriage;
  • Stillbirth (see the stillbirth risk calculator);
  • Sleep apnea; and
  • C-section complications, e.g., wound infections.
Łucja Zaborowska, MD, PhD candidate
Before pregnancy
You are...
Weight during pregnancy
Min. weight
Max. weight
Min. weight gain
Max. weight gain
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