Pregnancy Weight Gain Calculator by Week (IOM/NAM 2009 Guidelines)

1. Set your BMI category. Enter your pre-pregnancy height and weight to compute your BMI, or pick your category directly if you already know it.
2. Choose singleton or twins and enter your current gestational week, from 1 to 42.
3. Add your gain so far (optional). Enter your total gain to date and pick kg or lb; the tool shows whether you are within, below or above the IOM guideline range for your week.

Every figure is a population guideline range, not a personal target. It is not medical advice or a diagnosis.

During pregnancy, the body gains weight from the growing baby, the placenta, extra fluid and blood, a larger uterus and some fat stores. How much gain is healthy depends mainly on your weight before pregnancy.

The Institute of Medicine, now the National Academy of Medicine, published ranges in 2009 that link recommended total gain to your pre-pregnancy body mass index. These ranges are used by the CDC and by maternity care in many countries.

The guidance gives a range, not a single number, for each BMI category. A lower pre-pregnancy BMI is matched with a larger recommended gain, and a higher pre-pregnancy BMI with a smaller one.

These are population guidelines. Your own healthy gain can differ, which is why the result here is framed as a range to discuss with your maternity care provider, not a target to hit.

The calculator first finds your pre-pregnancy BMI category, then reads the IOM 2009 total range for it, and finally estimates the gain expected by your current week.

1. Find the category. BMI below 18.5 is underweight, 18.5 to 24.9 is normal weight, 25.0 to 29.9 is overweight, and 30.0 or more is obese.
2. Read the total range. Each category has its own IOM total-gain range for the whole pregnancy.
3. Add the first-trimester gain. The IOM assumes 0.5 to 2 kg (1.1 to 4.4 lb) in the first trimester for all BMI categories.
4. Apply the weekly rate. From about week 13, add the 2nd and 3rd trimester weekly rate for each week beyond 13.

The weekly rate applies only to the second and third trimesters. The expected range is capped so it never exceeds the full-pregnancy total.

These are the IOM 2009 singleton ranges. They are the only figures this tool uses, with kilograms and pounds as the IOM publishes them.

First-trimester gain for all categories is 0.5 to 2 kg (1.1 to 4.4 lb). It is an IOM modelling assumption, not a separate figure per BMI category.

Worked Example: Normal BMI, Week 20

Take a woman whose pre-pregnancy BMI was 22, so she is in the normal-weight category, carrying one baby at 20 weeks.

1. Total recommended gain for normal weight is 11.5 to 16 kg.
2. First-trimester gain is 0.5 to 2 kg.
3. Weeks beyond week 13 is 20 minus 13, which is 7 weeks. The weekly rate is 0.35 to 0.50 kg, so 7 x 0.35 = 2.45 kg to 7 x 0.50 = 3.5 kg.
4. Expected gain by week 20 is 0.5 + 2.45 = 2.95 kg to 2 + 3.5 = 5.5 kg.

So at week 20 a normal-weight woman would be expected to have gained about 2.95 to 5.5 kg, on the way to a total of 11.5 to 16 kg. These are ranges to discuss, not targets.

The IOM ranges are a starting point for a population. Several things change what is healthy for one person, which is why your provider adjusts the guidance.

Pre-Pregnancy BMI

This is the main driver. A lower starting BMI is matched with a larger recommended gain, and a higher starting BMI with a smaller one.

Carrying Twins

Twin pregnancies need more gain, and the IOM twin ranges are higher. The IOM labels them provisional and gives none for underweight women with twins.

Teenage Pregnancy and Short Stature

The IOM notes that very young mothers and women of short stature may need individual guidance, because the population ranges may not fit them well.

Timing Within Pregnancy

Little gain is expected in the first trimester, then a steadier weekly rate in the second and third. A snapshot at one week is only part of the picture.

Medical Conditions

Conditions such as gestational diabetes, high blood pressure or fluid changes can affect gain and its interpretation, and belong with your care team.

For twins, the IOM gives provisional total ranges by pre-pregnancy BMI. They are higher than the singleton ranges, and the IOM is explicit that they are provisional.

The IOM did not set a twin range for underweight women because the data were insufficient. If you are underweight and carrying twins, the tool says so and points you to your maternity care provider rather than inventing a number.

Because the IOM publishes only a total for twins and no weekly rate, this tool does not show a week-by-week expected range for twin pregnancies. Read more in our guide to pregnancy weight gain.

The whole guideline hangs on four pre-pregnancy BMI categories. The same cutoffs are used by the CDC and in general BMI classification.

The IOM uses one obese category for all obesity classes, with a single recommendation, rather than separate ranges for each class. You can check your own figure with our BMI calculator and read how much weight to gain in pregnancy by BMI.

At a Prenatal Check

Use it to see whether your gain so far sits in the IOM guideline range for your week, as a talking point with your provider.

Planning Across the Trimesters

The expected-by-week figure shows how gain is spread, with little in the first trimester and a steadier rate after.

With a Twin Pregnancy

See the provisional twin totals for your category, keeping in mind the IOM gives no range for underweight women with twins.

Alongside Other Pregnancy Tools

Pair it with our pregnancy calculator for dates, and our health calculators for related measures.

1. Using Current Weight for the BMI

The categories use your weight before pregnancy. Entering a current pregnancy weight gives the wrong category and range.

2. Treating the Range as a Target

The ranges are population guidelines, not a goal to hit. Your own healthy gain may sit outside the range for good reasons.

3. Expecting Steady Gain from Week One

Very little gain is expected in the first trimester. The weekly rate applies from about week 13, in the second and third trimesters.

4. Reading the Band as a Diagnosis

The within, below or above note is general guidance, not a diagnosis. Only your provider can interpret your gain in context.

5. Using Singleton Ranges for Twins

Twins have higher provisional ranges. Set pregnancy type to twins so the right figures are used.

6. Inventing a Twin Range for Underweight

The IOM gives none for underweight women with twins. This tool says so rather than guessing a number.

The arithmetic reproduces the IOM 2009 ranges and the expected-by-week model exactly. The limits lie in what population guidelines can say about one pregnancy.

This is an educational estimate, not a diagnosis or a personal target. Discuss your own weight gain with your maternity care provider.

It depends on your pre-pregnancy BMI. The IOM 2009 singleton ranges are 12.5 to 18 kg if underweight, 11.5 to 16 kg if normal weight, 7 to 11.5 kg if overweight, and 5 to 9 kg if obese. These are guideline ranges, not personal targets.

BMI is your pre-pregnancy weight in kilograms divided by your height in metres squared. Below 18.5 is underweight, 18.5 to 24.9 is normal weight, 25.0 to 29.9 is overweight, and 30.0 or more is obese.

The tool adds the first-trimester gain of 0.5 to 2 kg to the weekly rate for each week beyond week 13. For a normal-weight woman at week 20, that is about 2.95 to 5.5 kg.

The IOM provisional twin totals are 17 to 25 kg if normal weight, 14 to 23 kg if overweight, and 11 to 19 kg if obese. There is no firm recommendation for underweight women carrying twins.

No. The IOM applies the weekly rate only to the second and third trimesters. It assumes a single first-trimester gain of 0.5 to 2 kg for all BMI categories.

The tool flags it as below or above the IOM guideline range for your week. This is general guidance, not a diagnosis. Being outside the range can be fine for your situation, so discuss it with your provider.

No. The categories and ranges use your weight before pregnancy. Using a current pregnancy weight would put you in the wrong category.

No. They are population guidelines from the IOM. Your own healthy gain may differ because of twins, age, height or medical conditions. Treat the range as a talking point with your maternity care provider.

Your maternity care provider, such as your midwife or obstetrician. They can interpret your gain alongside your history, your scans and any test results.

No. The math runs in your browser. The Save button keeps results only in your own browser storage.

• Weight Gain During Pregnancy (singleton totals and BMI categories) (CDC; all recommendations attributed to the Institute of Medicine; accessed 2026-10-05).
• Prenatal Nutrition Guidelines: Gestational Weight Gain (full IOM 2009 table) (Health Canada; totals in kg and lb, weekly rate, first-trimester gain and twin provisional ranges; accessed 2026-10-05).
• Weight Gain During Pregnancy: Reexamining the Guidelines (2009) (Institute of Medicine and National Research Council, now the National Academy of Medicine; the originating report; accessed 2026-10-05).