Pregnancy Weight Gain Calculator
Free pregnancy weight gain calculator based on IOM 2009 guidelines. Enter your pre-pregnancy weight, height, and current gestational week to see your recommended total gain and whether you are on track.
Track your pregnancy weight gain against IOM 2009 guidelines by trimester.
Gaining the right amount of weight during pregnancy is one of the most important steps you can take for your own health and for your baby’s development. Too little weight gain is linked to preterm birth and low birth weight; too much weight gain raises the risk of gestational diabetes, caesarean delivery, and postpartum weight retention. This calculator uses the Institute of Medicine (IOM) 2009 guidelines to tell you how your current weight gain compares with evidence-based recommendations for your pre-pregnancy BMI category.
How Weight Gain Is Recommended
The IOM divides pregnant women into four categories based on pre-pregnancy Body Mass Index (BMI) and gives each category a recommended range of total weight gain plus a recommended weekly rate during the second and third trimesters.
BMI formula: BMI = weight (kg) ÷ height (m)²
IOM 2009 Recommended Weight Gain Ranges
| Pre-pregnancy BMI Category | Pre-pregnancy BMI | Total Recommended Gain | Weekly Rate (2nd/3rd trimester) |
|---|---|---|---|
| Underweight | Below 18.5 | 12.5 – 18 kg | ~0.5 kg/week |
| Normal weight | 18.5 – 24.9 | 11.5 – 16 kg | ~0.42 kg/week |
| Overweight | 25.0 – 29.9 | 7 – 11.5 kg | ~0.28 kg/week |
| Obese | 30.0 and above | 5 – 9 kg | ~0.22 kg/week |
These figures apply to singleton pregnancies. Twin or higher-order pregnancies have separate IOM guidelines (not covered here).
First Trimester vs. Second and Third Trimester
Weight gain is not linear throughout pregnancy. During the first trimester (weeks 1–13) most women gain only about 0.5–2 kg total, averaging approximately 1–1.5 kg depending on their BMI category. Nausea, food aversions, and metabolic changes keep first-trimester gain modest.
From the second trimester onward (weeks 14–40), weight gain accelerates as the baby grows rapidly, the placenta enlarges, blood volume expands, and amniotic fluid accumulates. This calculator uses weekly rates calibrated to each BMI category to estimate expected cumulative gain at any given gestational week.
How the Calculator Works
- You enter your pre-pregnancy weight and height — these are used to calculate your pre-pregnancy BMI and determine which IOM category applies.
- You enter your current weight and gestational week — the calculator computes how much weight you have gained so far.
- The calculator then estimates the expected gain at your gestational week using the first-trimester base gain plus the appropriate weekly rate for weeks after 13.
- Your actual gain is compared with the expected midpoint plus a tolerance of ±2 weeks of gain. If you are within this band, you are classified as on track. If you are below, you are classified as under; if above, as over.
This tolerance band is intentionally generous. Normal fluctuations in fluid, food intake, and measurement timing make week-to-week comparisons inherently imprecise. The goal is to identify meaningful deviation from expectations, not minor daily variation.
How to Use This Calculator
- Select your unit system (metric: kg and cm; imperial: lb and in).
- Enter your pre-pregnancy weight — the weight you were at before you became pregnant.
- Enter your height — used to calculate your pre-pregnancy BMI.
- Enter your current gestational week — the number of weeks pregnant you currently are (1 to 42).
- Enter your current weight — measured today or at your most recent appointment.
- Review the results — the calculator shows your pre-pregnancy BMI, recommended total gain range, your current gain, expected gain at this week, and whether you appear to be on track.
Example Calculations
Example 1: Normal-Weight Woman at Week 20
Inputs: Pre-pregnancy weight 65 kg, height 165 cm, gestational week 20, current weight 70 kg.
Pre-pregnancy BMI: 65 ÷ (1.65 × 1.65) = 65 ÷ 2.7225 = 23.9 → Normal weight category.
Recommended total gain: 11.5 – 16 kg at 0.42 kg/week.
Expected gain at week 20: 1.5 kg (first trimester base) + (20 – 13) × 0.42 = 1.5 + 2.94 = 4.4 kg.
Actual gain: 70 – 65 = 5.0 kg. Tolerance = ±0.84 kg. Range: 3.6 – 5.3 kg. → On track.
Example 2: Overweight Woman at Week 30
Inputs: Pre-pregnancy weight 80 kg, height 165 cm, gestational week 30, current weight 88 kg.
Pre-pregnancy BMI: 80 ÷ 2.7225 = 29.4 → Overweight category.
Recommended total gain: 7 – 11.5 kg at 0.28 kg/week.
Expected gain at week 30: 1.0 + (30 – 13) × 0.28 = 1.0 + 4.76 = 5.8 kg.
Actual gain: 88 – 80 = 8.0 kg. Tolerance = ±0.56 kg. Range: 5.2 – 6.3 kg. → Over.
In this case the provider should be consulted to discuss dietary choices and physical activity.
Frequently Asked Questions
Why does pre-pregnancy BMI determine the recommended range? Women who begin pregnancy underweight have greater nutritional needs and lower fat reserves, so they need to gain more. Women who begin pregnancy with obesity already have excess fat stores and face higher metabolic risks if they gain additional large amounts of weight, so their recommended range is lower. The IOM set these recommendations after reviewing extensive evidence on birth outcomes, postpartum weight retention, and maternal morbidity.
I am losing weight in early pregnancy due to morning sickness. Is that okay? Moderate weight loss in the first trimester is common and usually not harmful to the baby if you can maintain adequate hydration and nutrition. However, severe or prolonged vomiting (hyperemesis gravidarum) requires medical treatment. Do not attempt to restrict intake to lose weight during pregnancy. Discuss any significant first-trimester weight loss with your obstetrician or midwife.
I gained more than the recommended amount — what should I do? Do not attempt to lose weight during pregnancy. The goal is to slow the rate of gain rather than reverse it. Your provider can recommend dietary adjustments (reducing added sugar and ultra-processed foods, increasing vegetables and lean protein) and appropriate physical activity. Most evidence supports that walking, swimming, and prenatal yoga are safe for healthy pregnancies.
What about twin pregnancies? The IOM published separate guidelines for twin pregnancies: normal-weight women carrying twins are advised to gain approximately 17–25 kg total; overweight women 14–23 kg; obese women 11–19 kg. This calculator is designed for singleton pregnancies only. If you are carrying twins or more, ask your obstetrician for personalised guidance.
How accurate is the weekly rate used in this calculator? The IOM weekly rates are population averages derived from large studies of healthy singleton pregnancies with good birth outcomes. Individual variation is considerable — some women gain more in certain weeks and less in others, depending on fluid shifts, swelling, and appetite. Use the on-track / over / under classification as a general guide, not a precise clinical target.