Pregnancy Weight Gain Calculator

Enter your pre-pregnancy weight, height, and current week to see the IOM-recommended gain range for your BMI category and track healthy weight gain week by week.

How to Use

  1. Enter pre-pregnancy info

    Input your pre-pregnancy weight (kg) and height (cm).

  2. Enter current week

    Input your current pregnancy week.

  3. View results

    See the recommended weight gain range for your current week and overall pregnancy.

What is pregnancy weight gain?

Pregnancy weight gain is a normal physiological change driven by the growth of the fetus, placenta, amniotic fluid, increased blood volume, and maternal tissues. It is not about simply gaining or losing weight at random — the appropriate range is set according to your pre-pregnancy BMI.

Why is BMI the benchmark?

This calculator uses the guidelines published in 2009 by the Institute of Medicine (IOM, now the National Academies). Because energy reserves before pregnancy differ from person to person, the recommended total gain varies: 12.5–18 kg for underweight mothers, 11.5–16 kg for normal weight, 7–11.5 kg for overweight, and 5–9 kg for obese.

Gaining within the recommended range lowers the risk of low birth weight and preterm birth, while excessive gain raises the risk of gestational diabetes, hypertension, and macrosomia. Use it as a baseline for tracking your weight week by week at prenatal visits.

Calculation Formula

First, determine your IOM category from your pre-pregnancy BMI.

BMI = weight (kg) ÷ (height (m))²

The recommended gain for the 2nd and 3rd trimesters (from week 14) is as follows.

Maximum = 1st-trimester gain (2.0 kg) + weekly rate × (current week − 13)

Worked example

Height 165 cm, pre-pregnancy 60 kg → BMI = 60 ÷ 1.65² = 22.0 (normal). The weekly rate for normal BMI is 0.42 kg. At week 20, (20−13)=7 weeks, so the maximum = 2.0 + 0.42×7 = 4.9 kg, and the minimum works out to about 3.1 kg.

Frequently Asked Questions

How much weight should I gain during pregnancy?
The appropriate weight gain depends on your pre-pregnancy BMI. The IOM (Institute of Medicine) recommends 12.5–18 kg for underweight (BMI<18.5), 11.5–16 kg for normal weight (18.5–24.9), 7–11.5 kg for overweight (25–29.9), and 5–9 kg for obese (30+).
Why does the recommended gain differ by BMI?
If your pre-pregnancy BMI is high, your body already has ample energy reserves, so reducing additional weight gain has little effect on fetal development. Conversely, underweight women need more weight gain for the health of both mother and baby. Excessive weight gain raises the risk of gestational diabetes, hypertension, and other complications.
What are the IOM guidelines?
The IOM (Institute of Medicine, now the National Academies of Sciences) guidelines are the recommended standards for pregnancy weight gain published in 2009. Based on pre-pregnancy BMI, they provide a total weight gain range and a weekly rate for the 2nd and 3rd trimesters, and are the most widely used benchmark worldwide.
When does weight gain begin?
In the first trimester (weeks 1–13), regardless of BMI, you gain only about 0.5–2 kg slowly, with the main increase happening in the 2nd and 3rd trimesters (from week 14). This calculator also applies a gentle linear gain up to week 13 and then uses the BMI-specific weekly rate to derive the range afterward.
What is the weekly rate for the 2nd and 3rd trimesters by BMI?
Per IOM, the weekly rate is about 0.51 kg for underweight, 0.42 kg for normal weight, 0.28 kg for overweight, and 0.22 kg for obese. So for the same week, the lower your pre-pregnancy BMI, the larger the recommended weekly increase.
What happens if I gain too much weight during pregnancy?
Excessive gain beyond the recommended range raises the likelihood of gestational diabetes, gestational hypertension, macrosomia, and cesarean delivery, and also makes postpartum weight loss harder. If you consistently exceed the recommended upper limit, talk to your doctor about diet and exercise.
Is it a problem if I gain less than recommended?
Insufficient weight gain can increase the risk of low birth weight, preterm birth, and fetal growth restriction. If your gain is slow due to morning sickness, eat nutrient-dense foods in smaller, more frequent portions rather than severely restricting food, and consult your medical team.
Does this calculation apply to twin pregnancies?
This calculator is based on a singleton (one baby) pregnancy. For twins, the IOM recommended total gain is higher — about 17–25 kg for normal BMI — so multiple pregnancies should follow separate standards and individual guidance from your medical team.
Updated 2026 — WHO standards

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