Pregnancy Weight Gain, Explained
After reading this you can turn your pre-pregnancy BMI into the Institute of Medicine recommended total weight gain, work out the weekly rate for the second and third trimesters, and check whether your gain at a given week is on track.
The Pregnancy Weight Gain Calculator answers a narrow question: given your height and your weight before pregnancy, how much total gain does the Institute of Medicine (IOM) suggest for a single baby, and what does that imply week by week? The answer is a range, not a single number, and the range depends only on your starting BMI.
Here is the hook. Two people who both plan to gain "a healthy amount" can have very different targets. Someone starting at a BMI of 21 has an IOM range of 11.5 to 16 kg. Someone starting at a BMI of 31 has a range of 5 to 9 kg. The lower starting body mass buys a larger recommended gain, because the fetus, placenta and extra maternal tissue cost roughly the same in both cases, and the guidance adjusts for what the body already carries.
What the calculator does and does not do
The tool maps your pre-pregnancy BMI to one of four IOM categories, reads off the total gain range for that category, and spreads the gain across the pregnancy using a fixed first-trimester allowance plus a steady weekly rate afterward. It then multiplies that rate by your current week to show the gain you would expect by now.
It does not track your actual measured weight over time, it does not diagnose anything, and it does not adjust for twins, medical conditions, or a starting BMI that your clinician has already flagged. The ranges assume a single baby. Twin pregnancies use higher targets that this tool does not compute.
This is an educational estimate, not medical advice. Actual healthy gain varies with your history, and a professional may set a target that differs from the IOM range on purpose.
When to use it, and when not
Use it early, once you know your pre-pregnancy weight, to get a sense of the whole-pregnancy target and the weekly pace. Use it mid-pregnancy to sanity-check whether your gain so far is inside a plausible band. It is a rough map, useful for setting expectations.
Do not use it to decide whether to eat less, and do not treat a number outside the band as a verdict. Water retention, a large meal, and the time of day can move the scale by 1 to 2 kg on their own. If you are carrying twins, if you started underweight or with obesity, or if you have any pregnancy complication, the number here is not the one that applies to you.
The BMI categories and their gain ranges
Pre-pregnancy BMI is the ordinary body mass index, computed from the weight you carried before conception.
Here weight_kg is your pre-pregnancy weight in kilograms and height_m is your height in metres. A person 165 cm tall weighing 60 kg has \text{BMI} = 60 / 1.65^2 = 22.0, which lands in the normal category. The IOM total-gain ranges follow.
| Category | BMI range | Total gain (kg) | Weekly rate 2nd/3rd trimester (kg) |
|---|---|---|---|
| Underweight | below 18.5 | 12.5 to 18 | 0.44 to 0.58 |
| Normal | 18.5 to 24.9 | 11.5 to 16 | 0.35 to 0.50 |
| Overweight | 25 to 29.9 | 7 to 11.5 | 0.23 to 0.33 |
| Obese | 30 and above | 5 to 9 | 0.17 to 0.27 |
Turning the total into a weekly rate
Most gain happens after the first trimester. The IOM model assigns a small fixed amount to the first 13 weeks, then a steady rate for the rest. A common convention allows about 0.5 to 2 kg in the first trimester, then spreads the remainder over the roughly 27 weeks that follow.
Here total is the target total gain, first-trimester gain is the fixed early allowance, and the denominator is the number of weeks from week 13 to week 40. For the normal category, taking a 2 kg first-trimester allowance and the top total of 16 kg gives (16 - 2) / 27 = 0.52 kg per week, close to the published upper rate of 0.50. The expected gain by any week combines the two pieces.
w is your current week. Before week 13 the second term is zero, so the expected gain is just the first-trimester allowance.
A worked example with the demo data
165 cm, 60 kg before pregnancy, week 20
- Convert height: 165 cm is 1.65 m.
- BMI: 60 / 1.65^2 = 60 / 2.7225 = 22.04. That is the normal category.
- Total gain range for normal: 11.5 to 16 kg.
- Weekly rate: using a 1 kg low and 2 kg high first-trimester allowance, low rate is (11.5 - 1)/27 = 0.389 kg per week, high rate is (16 - 2)/27 = 0.519 kg per week. Rounded, that is the familiar 0.35 to 0.50 band.
- Expected gain by week 20: low is 1 + 0.389 \times 7 = 3.72 kg, high is 2 + 0.519 \times 7 = 5.63 kg.
So at week 20 this person would expect to have gained roughly 3.7 to 5.6 kg, on the way to a total of 11.5 to 16 kg by week 40.
Reading the result sensibly
Treat the output as a band with soft edges. If the tool says 3.7 to 5.6 kg by week 20 and your scale reads 4.8 kg, you are comfortably inside. If it reads 2.5 kg or 7 kg, that is a signal to watch the trend, not a diagnosis. What matters more than any single week is the slope: are you gaining at roughly the recommended rate over several weeks?
Weigh yourself at the same time of day, on the same scale, ideally once a week rather than daily. Day-to-day swings of 1 to 2 kg are mostly water and food, not tissue, so a weekly number is far more stable.
Common mistakes
- Using current weight instead of pre-pregnancy weight
- The BMI category must come from the weight before conception. Feeding in a mid-pregnancy weight inflates the BMI and pushes you into a lower-target category.
- Reading one number as a target instead of a range
- The IOM gives a range for a reason. Aiming for the exact midpoint is not more correct than sitting anywhere inside the band.
- Applying single-baby ranges to twins
- Twin pregnancies use higher targets, for example 17 to 25 kg for a normal starting BMI. This tool does not compute those.
- Comparing weeks across different conventions
- The first-trimester allowance (here 1 to 2 kg) is a modelling choice. A clinic using a different early allowance will produce slightly different weekly numbers with the same total.
Related tools
To pin down your week and due date, use the Pregnancy Due Date Calculator. If you are still planning, the Ovulation & Fertility Calculator maps the fertile window. For daily fluid targets, see the Water Intake Calculator, and to classify a blood pressure reading during pregnancy, the Blood Pressure Category Calculator uses the AHA categories.
Frequently asked questions
Why is the recommended gain lower if I started at a higher weight?
The fixed costs of pregnancy (the baby, placenta, fluid, and some maternal tissue) are similar across starting weights. A body that already carries more fat has less need for additional stores, so the IOM sets a smaller total: 5 to 9 kg for a BMI of 30 and above versus 11.5 to 16 kg for a normal BMI.
How much should I gain in the first trimester?
Little. Most guidance allows roughly 0.5 to 2 kg across the first 13 weeks. This tool uses 1 to 2 kg, then applies the steady weekly rate afterward. If nausea kept your first-trimester gain near zero, that is common and usually fine.
What weekly rate should I expect in the second and third trimesters?
For a normal starting BMI, about 0.35 to 0.50 kg per week. Underweight is slightly higher (about 0.44 to 0.58), overweight lower (about 0.23 to 0.33), and obese lower still (about 0.17 to 0.27).
My gain is above the band. Is something wrong?
Not necessarily. A single high reading can be water or the timing of the weigh-in. Look at the trend over three or four weeks, and raise a persistent pattern with your clinician rather than reacting to one number.
Does the calculator work for twins?
No. The ranges here assume a single baby. Twin targets are higher and depend on the same starting BMI categories, but this tool does not calculate them.
Related tools
Antibiotic Resistance Evolution Simulator
Watch bacteria evolve resistance — and see why finishing the course matters.
Blood Alcohol Estimator
Widmark BAC estimate and hours until sober.
Drug Half-Life & Dosing Simulator
Plot how a drug builds up and washes out of your body across a full dosing schedule.
Herd Immunity Threshold Explorer
See how vaccinating enough people stops an outbreak for everyone.
Ovulation & Fertility Calculator
Fertile window and ovulation for the next six cycles.
Pregnancy Due Date Calculator
Due date from last period, conception or IVF transfer.