BMI Calculator For Pregnant Women

BMI (body mass index) calculator for pregnant women

BMI Calculator for Pregnant Women

Created by Team y2calculate

Content written by Hamid (PhD)

Coding by Marcelino 

Reviewed by Sajid khan (head of content)

Fact checked 🔍✓

Published: February 16, 2024 → Last Updated: February 17, 2024

Table of Contents

What is Pregnant women BMI?

As you are looking for a BMI calculator for pregnant women, you may already be familiar with BMI. But what is Pregnant women BMI? Well, as women get pregnant, the overall weight of the woman includes not only her own weight but also the weight of her child. In this scenario, we subtract the baby’s weight and then calculate the woman’s BMI. Many people simply calculate the woman’s BMI without subtracting the baby’s weight and may conclude that the woman is overweight or obese. However, let me tell you, it is not her BMI; it is called apparent BMI, which is higher than her actual BMI. So, we have created this BMI calculator for pregnant women to accurately calculate the actual BMI of pregnant women.

Ranges of BMI

  1. Underweight:

    • BMI less than 18.5
  2. Normal weight:

    • BMI between 18.5 and 24.9
  3. Overweight:

    • BMI between 25 and 29.9
  4. Obesity:

    • BMI of 30 or greater

How do we calculate Bmi for pregnant women?

As I have already mentioned, we will simply subtract the baby’s weight from the overall weight of the woman and use the same formula given below.

BMI = kg/m2

But wait, how can we find the baby’s weight? Let’s discuss it below.

How to find the baby's weight to calculate the BMI of pregnant women?

As we all know, there are three trimesters in which a baby grows in the mother’s womb. Each trimester consists of 3 months, and the total duration of pregnancy comprises 9 months.

In the first trimester, the baby grows 1 to 1.5 kg per month, indicating a total growth of 3 to 4.5 kg in the initial trimester. For simplicity, we consider an average growth rate of 1.25 kg per month in the first trimester.

In the second and third trimesters, the baby’s growth rate is 1.5 to 2 kg per month. This implies that the baby grows 9 to 12 kg in these last two trimesters. The average weight gain will be 1.75 kg per month in the second and third trimesters.

Now, to find a pregnant woman’s BMI, we will subtract the weight of the baby from the overall weight of the mother and use the simple BMI formula given above. If you don’t want to perform this calculation, no need to worry. We have created a tool for it. You just need to enter the total weight for the month, the total height, and the month of pregnancy; the BMI calculator will handle this for you. Sounds easy, right?

One thing that I would like to mention, and of course, you might also be wondering about, is that according to the weight gain range we have discussed, the baby’s weight should be more than 14 kg at the time of birth. However, in reality, the baby’s weight is typically around 5 to 6 kg. Why such a big difference? Did we make a mistake? The answer is NO. We are not only taking into account the baby’s weight but also considering amniotic fluids, placenta, and other substances that support the baby in the womb, which contribute to such high weight.

Formula to calculate the BMI of pregnant women.

The formula to calculate the BMI of a pregnant woman by subtracting baby weight is:

\[ BMI = \frac{{(W - BW)}}{{H^2}} \]


  • \( BMI \): Body Mass Index
  • \( W \): Adjusted Weight (excluding baby weight)
  • \( BW \): Weight of the Baby
  • \( H \): Height of the Pregnant Woman

Example: Suppose \( W = 60 \, \text{kg} \), \( BW = 4 \, \text{kg} \), and \( H = 160 \, \text{cm} \). Calculate \( BMI \).


Using the BMI formula:

\[ BMI = \frac{{(60 - 4)}}{{(1.6)^2}} \approx 19.53 \, \text{kg/m}^2 \]

Therefore, the calculated BMI is approximately 19.53 kg/m².

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